Thanks for the comments guys.
Before I started building the kit, I sprayed the lower & upper hulls, turret and various other bits in Tamiya 'Bare Metal Silver' (AS-12). I cut up small pieces of an old bath sponge, put a little Humbrol masking fluid on the sponge, and dabbed it on the places that I wanted to show bare metal. After painting the required coats of olive drab, and leaving it to dry, it was just a case of rubbing the paint off with my finger wherever there were lumps in the paint caused by the masking fluid. Very easy to do, and well worth it for the effect it gives of making the model look like it's made of steel instead of plastic.
The 'splattering' on the front and rear, is real mud after a run in the garden. (Natural weathering?).

Richard L, I take your point about HL tanks running better with upgraded electronics. In fact that is what I intend to do very soon.
uhu, if I wanted to be Oddball, I'd need to find some 1/16th scale loudspeakers to mount on the turret.