An entry has appeared overnight from 2011 which I've tried to correct unsuccessfully and now there is a credit which has effected our VAT etc

Hi sales201, 

Thanks for your post and welcome to the Community.👋

Can I check where this entry appeared, was it in the For-Review tab of the banking page?

Often, if a tranasction has been deleted manually, this will cause is banking 'match' to become un-linked, moving it back to the for-review tab. 

You can check if any entries have recently been deleted by going to the Cogwheel > Audit log and then setting the filter settings to 'Show only these events' > Transactions and 'Show' > Deleted/voided transactions. 🧐