So it is safe to say that wheel spinning without engaging diff lock is a bad idea. After some quarry days it would seem that my transfer case is stuck permanently locked. I learnt after the case that it is a weak spot on a Discovery 1, or I guess any Land Rover / Range Rover with the same running gear. It certainly became clear after some loud banging whilst turning tight corners on tarmac. Anyways this video shows you how to swap out your box with a replacement.