Interesting tale of woe about your builder Bob but he will get his just deserts somewhere down the line. I believe in karma but sometimes as with the police it might need a little helping hand.
Cowboy builders deserve what they dish out and with spade fulls of interest too. They tend to have a very liberal relationship with things such as tax and honesty with the DWP both of which will be ever so grateful for a little helping hand. Surprising what some might be claiming when not entitled.
They also tend not to bother with little things like liability insurance but it might be worth looking into if not already done as it could be the way with the most merit to getting your money back.