@jane9,
Sorry, but I am in the US and our accounting rules could be different. Also, I am not an expert as I work in the mutual fund realm and the accounting may be different - I would hate to mislead you.
But from a high level viewpoint - we would simply reverse the initial entry and then rebook for the correct entry. From a US standpoint, the big issue I see here is if this crosses a fiscal year end (maybe quarterly too - I know in the US small businesses often times pay estimated taxes quarterly) - would taxes be impacted and how?
I would suggest finding some one familar UK accounting policies to make sure you record it correctly and take into consideration any other factors. Does the business have a public accountant that helps with filing taxes or similar things? They should be able to guide you.