Hi Richard
Apparently there were few if any Austin Tilly's supplied to the RAF most seem to have been Hillmans or Standards, however the vehicle may have been supplied to the Ministry of Supply or similar body for onward delivery to commonwealth forces which could account for it being in Australia but at the moment all of this is speculation. A small number of Austins were sent to Australia after WW2 to help fill a gap in the domestic market for tradesmen transport. They were sold through the Australian government who fixed the price to the retailers. Its possible mine was one of these which , I guess , may have never been recorded with the military.
Not much help with the DVLA
Thanks to everybody for the feedback so far, the search goes on !!
John