ouch, they will not have sympathy at all
Ignorance is no defence I'm afraid and if the DD's have not been going out then the £600 should still be in the bank, shouldn't it? You will have to make sure there is that amount in there ready for them to take - arrange an overdraft, the bank may be sympathetic towards you because it appears to be their mistake, the CSA won't be and you could end up with a long list of bank charges when you have a returned DD and you will sill owe the money anyway alongside huge bank charges