I wrongly categorised a credit card charge as a credit and need to correct. The error relates to a closed VAT period so I’m unable to edit or delete the original entry.

Hello Rival Ltd, 

Welcome to the Community page, 

You would need to do a journal to reverse this. Once you have done the journal you can then recreate the transaction so it goes to the right accounts. If you don't know how to reverse it on a journal you would need to speak to a trained accountant who will be able to advise you on how to do this in your account.