I picked up a copy of
Warpaint at Beltring. And very good it is too. It includes a full list of the 'T', 'F', 'D' and 'S' prefix numbers, so if anyone needs a suitable census number for their AFV let me know.
The War Dept. number is the same number as the T prefix census number, some manufacturers did apply the full "W.D. No. T.123456" and others had different and partial combinations of the letters, but all universally applied the numbers.