To be honest I would just keep clam and carry on. I really doubt you can do anything about it.

Why?

Proof. You say they took your code but whats to say they already had something similar in development. Without actually seeing the code changes you are just making assumptions (although it does sound suspicious).

Personally I would say take the moral high ground. Just forget it and move on. A lawyer will take you money and you will probably be worse off than before.