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Endorsed by Which? Trusted Traders

Core Sash Windows

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53 Reviews

Opening Hours
Today - 08:30–17:30
  • Monday 08:30–17:30
  • Tuesday 08:30–17:30
  • Wednesday 08:30–17:30
  • Thursday 08:30–17:30
  • Friday 08:30–17:30
  • Saturday - Closed
  • Sunday - Closed

What makes our traders different?

Double Glazing; Windows & Doors is an industry that Which? Trusted Traders has a strong focus on reducing consumer detriment in, due to the nature of it's work / audience of customers. Which? Trusted Traders carries out due diligence on all our endorsed businesses, and we want our consumers to continue to make the right choices about the companies they choose to carry out jobs, especially with renewable businesses. We have a useful article to let you know exactly what you can expect from our traders, find it under the 'advice' section of our website. Did you know Which? Trusted Traders work with a number of competent persons schemes to help raise standards within the home improvement and motoring sectors. For more information please check out our relationships page.


Core Sash Windows is a London-based double-glazing company specialising in the replacement and repair of traditional timber-framed windows and doors. We understand that replacement windows and doors are an important investment, and you obviously want to make the correct choice.

That is why we offer:

• Elegant and traditionally styled products
• Attentive customer care
• Punctuality and reliability
• Considerate workmanship
• Convenient and fully comprehensive service
• No need for Third Party trades

That is why we promise:

Our surveys are always performed by experienced carpenters instead of designated sales-people - there's no unnecessary up-selling and no high-pressure tactics. Our FENSA-registered installation service also includes the hand painting of your new products, traditional sand-and-cement pointing joints, and the repair of any damaged plaster. And our traditionally designed windows and doors come with a 10 Year Insurance-Backed Guarantee.

What we do

  • Glaziers

    • Sash windows
    • Timber windows
    • Window consultants
    • Replacement window services and repairs
    • Double glazing installation
    • Aluminium Windows
    • Double glazed units
    • Double glazing repair
  • Double glazing

    • Double glazing installation
    • Double glazing suppliers
    • Double glazing repairs
  • Carpenters

    • Joinery and carpentry
  • Doors

    • Bi-fold doors
    • French doors
    • Front door
    • Patio doors
    • Timber doors
    • UPVC doors
    • Orangeries
  • Draughtproofing installers

  • Specialised building trade contractors

  • More Services

    • double glazed timber sash casement windows and doors

Customer reviews

Why you can trust these reviews

All reviews are verified as being genuine and a true customer of this trader.

How can we be so sure?

100% of the reviews we publish have been through our stringent moderation process.

53 reviews
Customer Service
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Draught proofing front door
Customer Service
Very proactive - surveyor turned up on time as did fitter. Extremely tidy job and very effective"
Bridget1056 Posted on 07/12/2017 , work completed 07/12/2017
Unfortunately not able to recommend Core Sash Windows – 2 windows they installed 17 months ago have not lasted and now need to be replaced.
Customer Service
Core replaced 2 small single glazed sash windows in summer 2016, problems arose straight away. Despite 6 attempts to rectify, 10 different people from Core attending, they were unable to resolve to acceptable level. They spent total of 8 days, over a 10 month period working on 2 windows. Some of the issues (too many to list)-initial fitting–left a gap 5 cm wide, 2 mm high, could see straight through to outside. Wood severely splintered in the corner joints, paint on glass, holes in timber, paintwork lumpy, dents in putty. When the second fitter saw the newly fitted windows, they asked me twice if they were the old windows. On another occasion, strips of sealant were left hanging off wood, sashes made screeching noises when opened/closed. Manager did attend once, used his credit card to try & smooth out uneven putty, he admitted putty & paintwork was not good. Within 2 weeks of installation, the putty developed multiple deep cracks, within 2 months it was loose and moved. Core replaced the putty-their attempt was wonky, uneven & lumpy. A few months later, Core returned to paint the putty. Paint started creasing within hours, within days started cracking & flaking off. Their reasoning was that it ‘must have rained and been sunny on the putty’. Paint continues to flake & fall off. Core had the substandard windows made by another company. Core’s manager refused to take responsibility for windows. Ungenerous discount given doesn’t cover distress, upset & major inconvenience caused to me + I will have to pay for new windows.
Anonymous-15 Posted on 16/11/2017 , work completed 16/11/2017
The trader replied (on 11/12/2017)
Dear Anonymous-15 We agree that elements of the finishing by the fitter after Day 1 were in need of improvement, which is why we immediately returned to rectify the issues as soon as you brought them to our attention. As we explained when you decided to go ahead with single glazing replacement, putty will crack as it dries out, but the painted finish will fill these hairline cracks resulting in an unblemished finish. Also as explained, we could not paint the putty until it had fully dried, as putty can dry unpredictably which will affect the painted finish if applied prematurely. Nonetheless, at your request we replaced the putty resulting in further drying time before the windows could be fully finished. As you were dissatisfied with our explanation and resolution, at this stage we were mediating through the Dispute Resolution Ombudsman and Which?, and the discount you received was intended as a gesture of goodwill to repair our business relationship. We subsequently made several attempts to return to your property to paint the putty, as this was the only outstanding issue on the otherwise perfectly functional and finished windows, but unfortunately you did not allow us to do so, resulting in an impasse that we were unable to overcome. We're sorry that you are not entirely happy with the windows you have received, but we feel that we have genuinely done everything within our power during our business relationship. Yours faithfully. Core Sash Windows
Good quality windows and quick installation turn around
Customer Service
We recently had timber sash windows installed into our property. We ended up going for the basic fit as we were having a number of other work done to the property. This made installation much cheaper and affordable. In comparison the windows were much cheaper that competitors on the market and this was not reflected in the quality as we still got great windows installed. Highly recommended and great service by Dalan Rustemoglu!!!
Halil Posted on 07/08/2017 , work completed 07/08/2017
Sash window box refurbishment and double glazed sashes installed to the front.
Customer Service
Three years ago Core replaced some sashes and refurbished others on the back of our Victorian terrace. We also had replicated French doors and a back door replaced. I had no hesitation in asking Core to return for the front window. The information given was clear and no pressure or exaggerated promises were made. Core is able to offer a variety of options e.g. repair and refurbishment and replacement which is helpful if you value original features. I like the open accounting system and the option of insurance for the initial deposit. Financial arrangements were therefore straightforward and reliable. The team as before were professional, punctual, polite and honest and tidied up each day. The finish is excellent and being able to open and air the house more easily is wonderful. Both Daniel and Sean made sure to check that there were no issues. I am very happy and have recommended the company to a neighbour.
Ritten Posted on 27/07/2017 , work completed 27/07/2017
A less than satisfying experience - delays, incorrect charges and recurring issues
Customer Service
It started well, though there were delays and prices seemed on the high side. The team were competent and largely efficient regarding the windows though left the paper on one set of sash cords. The two doors and managements response were a tale of misleading pricing, leading to initial overcharging and lengthy delays because there was, it appears, a lack of internal communication within Core. The new basement door fitted well, but it took months to get the right colour hinges. The front door was much bigger negative issue. There were months of delays as a complaint to the original contact at Core was seemingly never passed on. The workmanship left paint bubbling, this was repaired but then bubbled up again. I was also, more seriously, but not clearly indicated on the estimate, charged for a full door refurbishment. I challenged this repeatedly and was only acknowledged as correct after repeated emails despite a full invoice requested - the discount for the one sided treatment, especially given the faulty workmanship was not generous and the door still required attention. Most of these issues could have been mitigated by more efficient customer service and better internal communications at Core and a better attempt at recognising poor customer management and service
Anonymous Posted on 19/04/2017 , work completed 19/04/2017
The trader replied (on 26/04/2017)
Hi Anonymous, Thanks for your feedback, and we apologise that you aren't entirely satisfied by your experience with Core. You've highlighted that many of these issues were either caused, or exacerbated, by poor internal communication, which we do acknowledge. Unfortunately, our internal resources have been stretched thin due to our company's rapid growth, but we understand that this is no concern of yours, and we will be improving in-house conditions to ensure that these issues do not arise again in the future. With regards to works carried out on your front door, you were originally over-charged as we had initially been contracted for a more comprehensive service than you were after, which we appreciate you have said was due to unclear indication on the estimate. This issue was then further compounded by our painter not informing us that he had performed the door repairs in line with your specific requests, as opposed to your accepted quotation, meaning that our office staff were unaware that you had received a downgraded service and therefore did not lower your price accordingly. Whilst there is no excuse for the poor internal communication, we would like to make it clear that the price you had originally was never intended to reflect the works you ultimately had performed on your door, and that these issues arose from miscommunication as opposed to malice. Kindest regards, Core Sash Windows
High Spec and Stylish Workmanship
Customer Service
I had researched the well known window and door manufacturers, but Core easily came out tops as they offered the most comprehensive service. The price is also reasonable given the outstanding care and workmanship. Ryan and Marco were full of tips and not pushy as this company has no need to be. Core produced a new front door for my Victorian terrace with a style in keeping with the era, though I chose a colour with a modern edge which compliments the front of my house brightening the exterior and interior. There are a large range of colours to choose from so you can create a bespoke look. The additional warmth created by the insulated door was immediately noticeable. And now I've commissioned core to replace all the sash windows in my home which will be natural wood inside (for a cottage like effect) and white on the exterior. I don't think other companies can offer that level of individual service.
Alison Posted on 13/02/2017 , work completed 13/02/2017
Superb professionalism and outstanding quality
Customer Service
We have been extremely impressed by the level of service and workmanship provided by Core. From the professionalism of the survey to the skill of the workmen involved with the installation and painting of the windows, it was been a flawless process. It may be a bit more expensive than other companies, but it is money well spent with quality that is second to none.
Jon S Posted on 03/02/2017 , work completed 03/02/2017
Very happy with our new windows!
Customer Service
Professional, friendly service and excellent workmanship. Reasonably priced, not the cheapest but reliable, clean and trustworthy. Head carpenter Ryan is responsive, helpful and a very nice chap. I wouldn't hesitate to recommend them.
Mrs B, Islington Posted on 21/11/2016 , work completed 21/11/2016
Very very happy with Core
Customer Service
I was so nervous about appointing someone to do our windows given some of the horror stories you here. However, I couldn't have been happier with Core. They are excellent quality windows, the builders always arrived on time and cleaned up after themselves. The communication and service with their office was always spot on. Payments were easy to keep track of and make through their customer site. And it makes the house so much warmer too - such a noticeable difference. Very happy to recommend to all.
Vicky V Posted on 09/11/2016 , work completed 09/11/2016
Fantastic work
Customer Service
excellent workmanship, amazing cleanliness and finishing off that goes beyond expectation - and great windows!
Which? Trusted Traders Consumer Posted on 04/08/2016 , work completed 04/08/2016