Has the work he botched been fixed now or is it possible to get photos of the problems as they have been discovered?
Would the guy who fixed it be willing to provide a statement to back up what he has said?
Have you got invoices, receipts, proof of payment etc... for what you paid the first guy and what for?
Might be worth an initial professionally done threat of further action and report to trading standard and such. Is he registered with any professional body you could threaten to report him to?
You could see if CAB would help you do that initially so you don't risk prejudicing any further action.
How much did you pay over to him and has it cost you extra to remedy his botched job over and above the work that needed doing in the first place and can you quantify it in money terms eg leak repair, fixing damp and painting over damp.
Could you have mitigated your loss in any way?
Make sure you keep receipts and such for everything connected with the work on the bathroom.
Could your household insurer help? Have you got legal expenses insurance on it?