Given the stated currency, I suspect the bank account in question is in the UK, so the criminal law wouldn't be entirely identical. But in any case, it wasn't

Given the stated currency, I suspect the bank account in question is in the UK, so the criminal law wouldn't be entirely identical. But in any case, it wasn't necessarily a crime -- someone (especially if elderly or impaired) might have accidentally used the wrong chequebook. It's only a crime to write a cheque on someone else's account if you do it deliberately. To me, the biggest issue here is simply "banks should not recycle account numbers".