Rhys
Go on the carpet cleaning forum, you probably get a better responce.
You have several options.
1. Get quotes. Its around £30 per room, £1.50 per step for stairs so this should give you an idea.
2. Dont get rug doctor. Go to HSS or a tool hire shop and ask for a Hot Water Extractor for carpets.
They will sell you a chemical that is safe on most carpets. You can get technical and the pros will say, Oh but you need to test the PH etc etc and other stuf.
Just test a small area of the carpet, so it does not run, colours can run but most modern carpets are designed to be cleaned. Do waste the water and dont put to much water on.
Hope this helps.
Dave