there is no PAYE of course because it's a one off
Right result but wrong reasoning. Whether or not a payment is one-off can have an effect on PAYE (particularly regarding the payment of casual workers) but it is not the one-off nature of this payment which would keep it outside the PAYE system. The reason for paying such a worker gross on production of an invoice would be because he was working under a contract for services rather than a contract of service.
Quote from Directgov:
If you have a ‘contract to provide services’ or a 'contract for services' with someone, then this is different from an employment contract and generally means you are self employed. A contract to provide services is an agreement between you and another person to undertake some work for them (for example paint their house). You do not become an ‘employee’ for this person - you just provide them with a service.