Hi,
I would suggest looking over the contract which you have drawn up together and then take this to your local citizens advice bureau,
Citizens Advice corporate website - Home
They will try and gather all the facts and hopefully give you some advice on how to proceed.
This sort of problem really annoys me as companies/people that you have described give the trades industry such a bad name making it very difficult for soletraders to be taken seriously.
Good luck,
Alex (plumber to be)