Alex said I needed to tap the gauge, which I did, but all the lights stayed on.
Definately come to the conclusion that the wiring for the LPG is a tad dodgey, as witnessed by Bobby, the guage on the dashboard lights up with all 5 lights indicating a FULL tank. The analogue dial guage on the tank now reads EMPTY.
I'll pop some gas in at the weekend and give it a whirl.
One thing it does indicate, that there is some other problem with the petrol system.