Richard, prior to the parts being changed today everything worked as it should and the switchable 4x4 was fine.
Hence why I am convinced the problem lies with what has been mentioned above, wires, pressure wotsit.... this is what I shall check at first light.
I did however discover that I have no TX box temp sensor, the wires are there but disconnected and there is just a plug in it's place.
Do I need one, if so where can I get it?
Thanks for your comments, I shall read the info on there.