Well, after explaining to him that there was nothing that I carry to fit a blind that could scratch glass he asked for me to go round and remove the half installed blinds and refund his deposit which I agreed to, then he suggested that I go and complete the job and he will pay me cash. I can only assume that someone else in the family has come clean on the scratch and he has backed down. Thanks for all the advice and help, I have found a gem of a forum and have learned a lot!