Notwithstanding that such "Guarantee"s are little more than a lure for unsuspecting consumers & worth not the paper their written on.
Firstly You have to persuade the offending institution that there Has Been a Mistake.
Then once they recognise that fact, they can still refuse, until the other Party (who they've just say Transferred your life savings to) also agrees it was a mistake.
I've had an Estate Agent happily declare that Money paid to them in error was Owed to them Anyway !
The Bank Did Not take reponsibility for that, even after acknowledging they had paid in error.
Then there was the mobile Phone operator who claim they were unable to repay , except as Call Credit !
I'd doubt if the Ombudsman could intervene before you'd been wronged anyway, and before giving the offender an opportunity to sort it out.
To be fair though, while RBS can be crafty, I've never allowed them to process a Direct Debit.