Trades people usually charge between £15-£25.00 per hour (dependining on where you are in the country). Depending on what they suggest how long they think they need to do the job, you can calculate thier hourly rate. They really should not charge more than £30 per hour. Ask for a break down of the prices and see if they would supply materials as well and how much they charge.
Slightly higher than average I would say (unless you are in London), that works out at approx 3 days work, which seems a lot for what you are having done. Are your providing paint and wallpaper ? Our decorator is £120 a day plus materials. He is very good and we use him loads as have BTL's. He is currently doing, 2 walls in the (largish) kitchen/breakfast (lots of fiddly bits around wall units windows, doors etc), cloakroom (typical size), 4 walls in lounge (23' x 14'), gloss front door. He is just painting walls no glossing or ceiling, 5 days work £600.
I actually think that's very reasonable. Most decorators (in my experience) charge about £500 per room (for all painting), so £390 for wallpapering, painting bedroom and ensuite (even though you're not having any gloss work done) is very good. We're in the NW by the way.
Thanks for the replies, we are providing paint and wallpaper. If I had the energy I'd get a second quote (!) but on the whole people think reasonable and he seemed very nice, which sways me. We are in Bucks.