So they still will not answer the elephant in the room question, namely; what VRM was actually registered against my payment?
Lead POPLA by the nose; point out that the machine will not allow payment if no VRM is registered - hence why the answer to your question is so important. Point out that you have challenged PM on this same point several times but they refuse to provide you with the details of the VRM recorded against your payment. This is deliberate behaviour on the part of PM because they know that there was a payment error on their machine which they are now making you responsible for.
It is not the client's role to mitigate against failure of the firm's payment machine and any term or condition which suggests this is unlawful.