Oz don't have a 20% VAT rate, so its definitely not Oz VAT. The only thing I can think of, is that they are VAT registered in the UK? Much like if you sell over 100,000 euros worth of goods/services to Germany, you have to register for German VAT? Is there anything resembling a UK VAT number on the invoice? Have you asked them what would happen if you gave them your clients VAT number? If it was RC, there would be no VAT charged, with a note saying RC was to applied.. I think!