Hi Richard,
Looked into various different routes but found this way the easiest to sort out. My Mot supplier is very good and is quite willing to re-invoice. He only has to give one invoice listing all the previous invoice numbers that I have to pay the difference on. I get a £6.15 discount on Mots at the moment, yet the Vat element is £7.50 so you have an immediate saving of £1.35 per Mot by re invoicing rather than paying Customs and Excise. The other point is that all the times that my supplier has borrowed specialist tools ect from me, which is quite often, I can invoice him for to recoup some of the payment.Obviously everything depends on your relationship with your supplier and his co-operation.
Mark M