Thanks Richard! I now suspect that the 1928 Scammell 6 x 6 whilst appearing similar to the FAT, Pioneer? FAT in the IWM photo was a production one. I kept believing I could see the huge front axle construction that the 1928 and 1929 6 x 6 prototypes had, well-placed in front of the rad.
The Trials repprt also mentions an order for four more Scammell 20-ton transporters, as per a 1932 pilot but with Gardner diesels. There is a '32 photo of a 20-tonner articulated Scammell Pioneer with Medium Tank Mk 11 transporter onthe IWM site.
I am loathe to suggest that this is the tractor referred to.