Even though they have broken the data protection act by sending your banking details to a third party. I dont think you will get very far with any reddress.
Prehaps if cancel your contract with them, then they might listen a bit more. After all they've broken there part.
Prehaps concider having a word with your bank so that they can give you a new account number, just in case.
And give the citizen advice a bell, to see what they say, about your rights and what you may expect.
When British Gas sent me banking details of one of there customers (without there address) not only did we tell BG, but also e-mailed the bank involved (worked out where abouts by sort code) but no-one was interested, so I very much doubt that the persons involved even knows what has happened (I await to see if we get there gas bill!!!!)