After reading advice to Zeek, I decided to go with a local d/g company. I spoke to one sales guy today, who seemed helpful but not pushy, and quoted me slightly less than the previous company.
Then he asked me what I thought and I said (honestly!) that it was more than we'd hoped but maybe we'd been unrealistic. He then said that if we'd let him put letters around the other 100 flats in the block saying they were doing ours, that he'd bring it down to just above what I'd hoped for (nearly 20% off). He said it was cheaper for them to do this than put an ad in the local paper.

First reaction: Great!
Second reaction: Worry...

What would you do? I don't have to decide in any rush as the offer doesn't expire, so it's not one of those '2K now but only if you sign NOW' scams. They don't want a deposit and are FENSA.
Any reactions would be much appreciated as I'm a bit nervous about this having heard horror stories about A****** etc!

I know a couple who have done this, but it was having a sign outside for a month and they got loads of money off too.

It's just advertising for them. I have to say, that gives me hope too, because it's kind of a way to check references. If a tradesman is proud enough to put his name to something you can actually go round and have a look at then that's a damn fine thing. I've seen it for block paving too, gone past thought, ooh, they've done a nice job Mr Bloggs, whereas Mr Cowboy could have done a rubbish job and not put his name to it.

Does that make sense? All in all, I don't think it is a con, it's advertising and a public way for him to show his work to potential customers and he wouldn't do that if he was cr@p.

Thanks Trish that's what I'm hoping, especially as there wasn't any pressure to sign anything then. I've got a few other quotes coming so we'll see how things compare.
The company is based just outside London, so I think they're keen to get more London jobs without paying London advertising rates!

Rach-------There is UPVC Double Glazing and there is Contract UPVC Double Glazing units. Make sure you get the right specification and not buy Contract D/G. The difference in price can be 25% per window unit.
If you can, try and use a supplier who is actually making the units on-site and not sending away for the manufacturing.

Hi Melboy

Thanks, but I don't quite follow - is the Contract cheap but nasty? What's the difference? To find out do I just ask the companies 'Is it Contract UPVC?'

This company makes units on-site and has said we could come and look at them, which I would do before accepting the offer. Even though it's South of the river!

If they are making units on-site than that is a good sign that the quality is up to standard.
Contract units are normally inferior in as much that the welding of the UPVC Profile during construction is different or the profiles are not the best available and the window or door furniture may not be as good a quality as it could be.
There may an 'expert' on the Forum who can give you much more detailed information perhaps than I can. My knowledge is from what I have been told. ( I know the Owner of a UPVC Manufacturing Company--Very good for discounts!)

Rach-17, I've been in this game 33 years. The offer sounds fine to me . If he wasn't pushy, and made that offer, I'd say it was a genuine discount, and his explanation about advertising is OK. DG companies are always after sales "leads", and if someone sees the sign and approaches them, not only is it a relatively cheap lead, but it's not likely to be a time-waster either. However, get them to show you a sample window, and details of guarantees etc. Whatever salesmen say, the normal situation is that the white PVC framing is guaranteed for 10 years ( by the extrusion Co. ), but the fittings - handles, locks etc, are only guaranteed for 1 year by the manufacturers; the glass units are guaranteed for 5 years.Window companies never tell you this, and I've often seen them using the variations as an excuse not to honour guarantees. All frames should now carry a date mark, likewise glass units. This stops people claiming from the wrong source. If these are not done, the chances are they aren't quite so good. You did say though that they are FENSA registered, so these things should be covered.Incidentally, another ruse is to offer customers £50 for recommendations which lead to a sale - i.e. if your mate also wants windows, and you pass on the lead, and it becomes a sale, you get £50. If he's a good mate, you could split it !

My gut instinct would be to trust the offer. If the company are going to potentially be advertising their services to 100 other people, using your property as their 'advert' then they are more likely to do a very good job. They're not going to want you telling everyone in those 100 properties that they are rubbish.