re the cashed off table - why not just wait till table 6 wants to pay and give them the bill/receipt?
alternatively, just void off the items, [no table required - neither the accounts or stock control care which table the cash came from or products went to] and finalise via the method of payment used on the erronious bill.

It's already been rung through- that's the problem. I cannot void off the items after the table has been cashed off.

I need to be able to delete a 'paid' bill after it's been rung up and cashed off.

Is there a refund facility?

To Do that turn the key to Void, then select each PLU from the incorrect rcpt.

why would you want to?
the correct diners at table 6 will want to pay at some point?

failing that, if its cashed then the table is closed, just void the items off in void mode or refund the items off in register- without opening a table