Making the invoice reference field a mandatory field when raising sales invoices in the sales ledger

Our clients raise their printed invoices within an external SOP system they raise the sales invoices in Sage 200's sales ledger We weren't aware of this, but you can tab over the reference field & post the invoice - WITHOUT an invoice number. We've raised this as a bug, but it has been declined saying it isn't a legal requirement as no document is produced, which we dispute. Therefore we would like to log this as an enhancement request