We requested quotes from three DG companies. The first a small local installer, which turned out to be the cheapest/ best spec..
Followed by two quotes from a couple of the larger installers. Both of these initially put in much higher quotes then found strange reasons to reduce their quotes as time passed. Both of these seemed to be just salesmen on commission, neither very knowledgeable and both extremely pushy. One even asked if we could afford their quality of DG - WHAT? He had to be invited to leave, it seemed he was going to sit there until he got an order.
So back to the first one who had obviously installed DG himself, never made a nuisance of himself, never pestered, came recommended and just offered one quoted price. He had also quoted for better than the basic spec. unasked and took the trouble to explain his reasoning for the quoted opening styles for the 13 various windows.
So if you might be considering new DG, my advice would be forget the national companies advertising on TV - just find a small local company with a good reputation, they don't need to advertise.