Incorrect VAT treatment on invoice spotted - after VAT return submitted

Hi,

Invoices for services delivered to USA mistakenly included VAT at 20%. The invoices have been paid and the corresponding VAT return has been submitted.

In order to correct this, I think issuing credit notes and then new invoices with the correct VAT treatment would seem logical, so that the next VAT return includes a credit for the mistake.

I'm unable to do this as the original invoice cannot be deallocated due to having been in a VAT return.

Any ideas what's the best approach to resolving, please?

Many thanks