I've just had a uPVC front door and side panel, both half-glazed, installed by GAP, whom I'd previously used for some windows and back door. They charged something in the region of £750 though I got a hefty discount by getting the patio doors done at the same time so it's hard to prise apart the two prices.

Ramsey Glass came in a hundred quid cheaper and also have a good reputation. But I'd already used G&P, so decided to use them again. Also, you might be outside Ramsey Glass's catchment area.