I didn't haggle but that's because the chap I liked best was also the cheapest and it was cheaper than I was expecting
What are the quotes like, and how many are you having done? If it helps I'm in Oxfordshire and had 9 windows, 3 exterior doors and 1 set French doors done in white uPVC (I know, I know, but we're mid-terrace and everyone else has them) for £5200 (A-rated, 10-year guarantee). And they fitted a new cat flap for free.
The other quotes were £6000 and £8000; both local firms rather than Anglian or anyone like that. The other two, as well as being more expensive, I didn't like at all and they weren't going to be the ones fitting them whereas the other chap did everything from sales to surveying to fitting.
No - we managed to knock 36% off the initial estimate (triple glazing though), and then get them to agree sandblasted windows for the bathrooms and a more expensive finish for the front windows as well when they wouldn't go any lower. They weren't cheap in the first place admittedly, but we'd used them before so knew the workmanship/after sales was top notch and didn't mind including that in our price considerations.