I don't know if there is any way to get the plumber that did the repair to go back and make the job right or not. The fact that you no longer own the house shouldn't make any difference, but I'm no legal expert. It also depends on what, if any, guarentee was made on the work. I'm thinking that even if you could eventually force him back, it would take time and you can't take time. You have to make the problem right even if that means paying another plumber to replace the shower completely. If you don't fix it promptly, you could be taken to court and forced to do it, so better do it now without having to pay lawyers as well as plumbers.