Vitruvius. I have had a similar intermittent and then permanent problem with a newish Smiths 3 in 1 guage. I began at the fuel tank, removing the sender to check its function; then I connected a spare single fuel guage to the sender and live feed to eliminate the sender; then that fuel guage to the wiring at the dash and all worked as it should so I assumed that the problem lay with the new 3:1 guage. Eventually I tightened all the small brass connectors on the back of the guage and the fuel level indicator sprang into life. I have my fingers crossed. It's worth checking you guage connections even if as mine, they appear tight already.
