Hi Paul,
Your truck is simular to ours. I dont drive it as I have no HGV, but thr two guys that do have both stated there is a knack to the gear changes. As has been said in 1 of the other threads there is a brake on the clutch, so you only need to press it down a fraction of the way for normal gear changes.
Ours was definately a tanker, and was changes over by a member on here, I belive from what we have found out that all the electrics go through breakers, and everything is wired positive and negative, not a negative earth.
If you find a data plate let me know where it is as I would like to find the history of ours, but we havent found a data plate with the military reg number yet.