How much should I pay a friend to paint and decorate my house?

Every week a Guardian Money reader submits a question, and it's up to you to help him or her out – a selection of the best answers will appear in Saturday's paper. This week's question I'm thinking of paying my out-of-work best friend to decorate my house, as a bit of a favour. What's a fair rate to pay, and what if it goes wrong – he doesn't usually do painting. Is £70 a day, cash in hand, fair? We live in London. Or should I just get a proper decorator in?