I got three local companies to come and quote for my windows plus one slightly larger one (but also fairly local). My research told me that there are only two companies making the Upvc extrusion that they make the frames from so basically all the companies are supplying the same product.
Anyhow the quotes i had ranged from £1300 to £6500 (for 6 windows). In fairness to the ones that came round non of them did the ringing their boss to get better price trick, they all gave me a price and that was it.
The most expensive quote was for externally glazed frames so that was a non starter, the others all quoted for esentailly identical windows so i went with the cheapest. Four years later i'm still very happy with them.
The secret is to know what you want before they turn up and don't let them baffle you with bull. I did drawings of what i wanted including opening lights and measurements etc so all they had to do was add it up and give me a figure.
Personally i'd stay well away from national companies based on my experience.