[cut for dullness]
The update is - but, first a recap:
First quote for replacing five sash windows with uPVC: Big company (Anglian), lots of attention, driven out to see a show house, terrifying amount reduced in the same breath by 50% discount, and then further by Finance, £8,630.
Second quote: Smaller firm (all covered and insured by the same regulator though), quick, businesslike, gave me a quote a few days later for £4,200.
And since then, largely a failure to get a third quote - every fortnight or so I'd go onto checkatrade and look for glaziers who came up when you enter my postcode, and inevitably they'd be based way out of town, but had added 'London' in the pretty rough filter they'd been given. So I'd ask for a callback, and not get one, and then the general end of year business turned up.
And then New Year New Attitude (the attitude in question being "man, fuck this bullshit") - the Sunday before last I went back, picked 4 (the maximum you can ask for a callback), and three (3) of them responded! So I now have five (5) quotes!
Wednesday was Advanced Glazing Systems (I know, but all the good names are surely taken years ago), who came over and was very charming, a quick measurement and a chat - took pains to point out that one of the things to look out for is the cavity in the panels where the weights and cord were (it's all springs these days), which they take care to fill in to avoid noise and heat leakage. On a related note, he recommended that whatever I do, have ALL of the dust sheets in place when the old windows come down - he's seen the cavities up to 3/4 of the way up full of dust. He mailed me the quote later - £4,150.
Thursday was Homeglaze, a big enough operation (the person who booked my appointment is not who answered the phone to reschedule it is not who turned up). He's heard of Anglian, and mocked the "Big quote and immediate discount", but he also had a finance discount and a February discount (the workmen get paid anyway and apparently winter is a slow period?). But after all that it's still £8,664, and he didn't even have a brochure to demonstrate their technical superiority (he had a sample window - it looked like all of the other sample windows). He was a curious figure, he reminded me a bit (and I'm aware this is harsh) of Shelley from Glengarry Glen Ross / Gil from the Simpsons - pushing a rock up a hill using willpower he doesn't have.
And then on Friday I sent a nagging text to Taylorglaze, who had come in on Tuesday (similar to the second quote - quick, businesslike, took the requirements and left) - they grudgingly sent on the result of their deliberations which was £4,980.
I'm still a little surprised by there being two bands of price, but then the larger two are from much larger organisations. I am hoping that the £4k ones are as okay as they seem, if not they're insured for 10 years and I should in a position to shell out the £8k without as much worry by that stage.
For peace of mind I think I'm going to pick AGS I think, not just for the lowest quote but because they had a good salesperson and decent reviews on Checkatrade. And then soon I can stop thinking about this which will be nice.
Things standing between me and stopping thinking about this: AGS sending someone to survey properly, me booking dates, crucially and terrifyingly me talking to downstairs about possibly allowing access to their back garden.