Whether it's reasonable or not, you need at least another two quotes, not just for comparison, but also for potentially highlighting alternative options or problems you may not have considered.
We've just had a small back porch replaced, straight swap of old PVC/DG to new, exactly like for like, no electrics, no plumbing, no plastering - just "making good". We had about 8-10 firms round to look at it and quote as we couldn't find anyone we were confident with. Lowest quote was £2,500, highest was £6,500, but that was a small porch, something like 2 metres by 1 metre. In the end, we didn't choose on price, we chose on other aspects, such as how many workers they'd put on the job, therefore how quickly it would be done, and also how thorough they were with the quote, measuring, etc., as we didn't want a sloppy bodge-it firm.
About 5 years ago, we had our main conservatory replaced, about 4 metres by 4 metres. We had triple glazed/coated windows around and a glass, coated roof, so all anti-glare, best insulation, also lots of opening windows, etc. That was £10,000 done by a local independent firm who turned out to be absolutely brilliant in every aspect. We'd had quotes up to £25k from some of the cough specialist bigger firms.