Get your bank to refund you under the direct debit guarentee. Quote that word for word and get a supervisor if they shilly-shally about.
I expect some numpty somewhere has entered an account number incorrectly rather than it being a fraud. In fact, I would strongly recommend you do not suggest it's a fraud as they will pass you to another department and take longer to sort it.
Your bank should have sorted it out the minute you rang. Make a complaint to them about how they've handled it.