how can i wire up my lpg system to run on the same guage as the petrol, are there any docos anywhere. all help is useful, thank you

Does Ray know anything about it?

Well a little bit.
You need to wire a 5 pin relay up:
pin 30: fuel gauge line
pins 87/87a: gas sender line/fuel tank sender line
pins 85/86: earth, fuel changeover switch.

You need to also check with your gas installer whether the LPG tank sender is compatible with the fuel gauge, and not just the LED gauge.