What should I do if I paid a DIY guy to do a job and it has not been fixed?

I saw an Ad. in a local magazine offering 'Handyman Services' so I called him out and asked him to see if he could fix my leaking porch roof. He got up there for a while and then said it was done to the best of his knowledge and I paid him �100. A week later it was dripping water again into my porch. It is still dripping. He has been back a couple of times and just looks at it and says wearily "well, I don't know how the water is getting in there". I'm sure he has done it to the best of his ability but he has not offered me my money back. If he doesn't give me my money back, what should I do?