Ok, I couldn't wait and got wet
Did the testing of the gauge and sender, everything looked OK.
I decided to wire the earth point (that I thought was for the instrument light) on the gauge to earth and hey presto she works!
I've never come across this before. All the gauges I've used have a live/hot terminal and one to the sender, then the senders earthed. The only other connections are for the light.
So why three wires?
Thanks guys for you help.
PS; is there any way I can wire one sender into two gauges, the analouge one and the multi-function Tachometer?