A quick answer is that I don't know! but I'll try to help guide you in selecting the right upvc window company.Choosing a firm to buy your replacement upvc windows from is difficult let me explain with this example.
When I was away on holiday recently we decided to go to a restaurant the main street was full of restaurants advertising high prices and they were packed with tourists.We walked a bit up into the side streets and found a small place also packed with customers the difference was they were locals ,we enjoyed a great meal at a lower price.
The message is look past the easiest obvious choices do a bit of homework and your family and inlaws will be happy.(inlaws maybe) That's easy said when buying something you know about like food but when it's something like a upvc window it's tricky so I've put down some things that will give you a good indication.
Price of the window quote
Firstly get at least two quotes and preferably three this should give a good indication of the market price if one quote is 50% more than the other two there having a luagh or have made an error.
Compare the window specifications
Make sure that the lowest window quote is not for a very low specification window with few features or importantly includes things you might have to pay later for ie window cills the other thing is to make sure you are comparing apples with apples ie are the specification for all the quotes the same?
Do they sound and act professionally?
A tricky one this as some of the slickest and definately likeable people I ever knew worked in double glazing!.Instead focus on how they deal with your enquiry do they respond to your questions promptly and correctly Ie not fob you off or try to spin you a line as to why XYZ company is rubbish.A company that tries to rubbish another by slagging them off is best avoided.
testimonials what do there customers say?
Its more than worthwhilst looking for feedback from other people who have used the windows before and most window companies should be able to supply you with at least 50 people who have bought there windows and are delighted.If they can't or the testimonials sound like your mum wrote them avoid.
Do they manufacture upvc windows or buy in ?
I have a "friend " who works in the upvc window industry he does alot of business he buys from where ever is cheapest and sell it many of the firms he bought from no longer tarde sorry Simon but people don't want to be let down.So always buy directly from the window manufacturer.
How long have they been in Business for?
Think about this "more than half of all companies fail in the first five years of business." (source http://www.questia.com) Its vital that they don't run off with your money before you get the goods and are also around to honour any work under the gurantee.Personally I would avoid dealing with any upvc window company with less than 10 years trading.
Get a Guarantee that's insured
If the firm do go bust you have the insurance company to fall back on many window companies issue there ten year gurantees these are worthless once the company ceases to trade.
So there you have it had to find a top notch eatery abroad and also what to look out for when buying upvc windows online