Hello there, Pauline Dudley.
You'll have to make an adjustment for the transaction that shows as unpaid. This is to correct the reporting method. For this process, it would be best to reach out to your accountant so they can guide you what accounts are to be used. You can take a look at this article on how to make an adjustment: Adjust your VAT liability.
Also, they're the best support that can decide whether there's a need to delete the other returns from 2015 and onward.
I'm always here if you need more help!
