Re: What good could they do with it
Trouble with that is, your account number and sort code are sort of "out there" anyway. Any organisation with whom you've ever signed a Direct Debit, or anyone whom you've ever sent a cheque has those details.
The thing with DDs is the Direct Debit Guarantee https://www.directdebit.co.uk/DirectDebitExplained/pages/directdebitguarantee.aspx should, in theory opffer some protection. That assumes that you check your account details regularly!