You are free to charge anything above £10 a night for Dog Boarding (24h). Sitter prices may differ considerably, depending on the area you live in, the variety of other services you provide (e.g: pick up and drop off for dogs) and also on how competitive you wish to be.
If in doubt, we would highly recommend you browse through www.dogbuddy.com as a Dog Owner, and go through a few profiles so that you can get a good indication of the average price range around your area.