Here is a summary of this mornings problem solving activities:
1 Upside down under the dash to try to see if anything is loose. Nothing looked out of place but solved another problem. There is a large plastic cylindrical tube that the harness goes into, it was rattling about behind the wiper motor, moved it and tie wrapped, annoying rattle from behind the dash now quiet.
2 Removed boot floor to get access to the top of the tank
3 Took the terminals off the sender unit and shorted them together - Fuel Gauge goes to the full condition: Conclusion Fuel gauge OK
4 Removed the sender unit from the tank and tested the resistance with multimeter: reading goes up and down consistant with the position of the arm: Conclusion Sender Unit OK
5 So if gauge and sender are ok unless it is an intermittent problem in the harness it should work
6 Put it all back together and it worked, but while screwing down the boot floor it stopped working. so lifted the boot floor again
7 on the female spade connectors I pinched closed the terminals a fraction. This appears to be the root cause of the problem as I put it all back together, went for a blat and all works OK
Problem Solved