A rounding issue has occurred on a line where the amount has been manually adjusted. Please delete the line, add a new line, and try again

Okay, I've managed to solve this 1 myself.   If anyone else needs the method this is it.....

1. Purchases
2. Create Supplier Payment
3. Your Supplier Name from the bill
4. Payment Date
5. Tick off the invoices the payment is applied to
6. Save
7. Then go to the Banking Transactions screen
8. Match your bank transaction to the 'payments' for that transactions
9. Save

And fingers crossed, you don't get that pesky, nonsense message, because for whatever reason it now matches.