Thanks everybody,
I did all the things you suggested and the sender checked out OK.
So I pulled the dash out to check the wires to the gauge and........it seems i did not connect one of the wires to the back of the gauge
It ended up being lucky that I pulled the sender out though as the rubber I used to seal the tank was disintergrating. I'm sure I purchased fuel resistant rubber. Does anybody know where I can get some actual fuel resistant rubber?