I have removed the gearbox oil cooler, I found the front part of it to be full of dirt, leaves etc. I took it out, cleaned it and tested it with an air compressor, it seems to be fine and not blocked at all. Also the radiator fluit is fine.
I found a garage that deals mainly with range rovers near to where i am, he plugged his machine and said that its the transfer box, as its showing a fault with the 5th gear. He is charging me £380 to supply and fit a second hand transfer box.
I dont know if I should go ahead with this or not.