Hi 7777, thanks for your reply.

Correct, the truck is NOT overheating.
We have checked resistance / short circuit through all of the wires relating to the temperature guage system and have indeed already by-passed the trucks wiring harness like you suggested. The new guage does exactly the same as the old guage (so doubt very much it's the problem).

I was hoping someone from Hyster might see this on here and could tell me the correct resistance range of the sender (so we can test it properly on the truck) and the correct resistance across the guage (again so we can test it to eliminate the problem). Hyster in the UK (now Briggs) don't appear interested in the problem....