It is fraud and they will notice over time, best to come clean and be ready to immediately repay any overpayments they gave you. The last thing you want is for it to drag on then you get a phonecall out of the blue telling you to attend the jobcentre the next day and bring 3 months bank statements, then it's too late. (they absolutely do this, and they're trying to push to just have access to your account info directly)