Still waiting for my Series 1 to cross the Nullarbor! but have tracked down some numbers (from James Taylors book - Original Land Rover Series 1) that help:

CHASSIS Number 113800295 - '58 RHD SWB 88''petrol CKD

Engine Number 170605678 - (a typo!) - 1956 Model (an engine change at some point)

Gearbox 111802161 - 1958 model petrol

Thanks for all your replies.