Adam from WW2talk forum has located a copy of the 21st Army Group Admin History on line and it is fascinating reading. It also gives the definitive answer to this question - They were Austin K5s, mentioned in relation to September 1944
"
(c) vehicle maintenance
During this intense period of activity the maintenance of vehicles inevitably had to be reduced, but partly due to the majority of vehicles being new no serious ill effects ensued. A major fault occurred in the engines of K-5 4x4, three-ton Austins, 1,400 of which, as well as all the replacement engines, were found to be defective and to have piston trouble."
The full report is here :-
http://www.movcon.org.uk/History/Doc...CHS%200514.htm
Rich