Thanks for that Richard, looks like the civvy version then,although still a great truck no doubt.
No, as Richard wrote it is an ex Royal Engineers SLDT with a Perkins 350/Fuller 9 speed and has been fitted with larger (Foden?) wheels and tyres to improve the top speed.
The standard engine for civilian S26s was the 14l Cummins 350 which although of similar power output had considerably more usable torque.