Decorators generally charge for the size of the walls, i.e the amount of wallpaper/paint they will have to use, and the amount of time it will take them to prepare. When we had a quote for 4 days work (no preparation needed but loads of woodwork!) for our large hallway we were quoted about £600. Most people I know use people that they have heard of through word of mouth, local retired people are popular as they are cheaper. We ended up doing it ourselves in the end, much less hassle!
Good luck
Hayley