First of all, are you getting the engine properly up to temp before switching to gas?
You say 'round town' so presumeably short hop work, and if the motor is cold, evaporator could be freezing, so the system isn't boiling the LPG to vapour, and consequently you are sucking neat liquid into the engine, in the sort of volume that should be delivered as a gas.... ie a heck of a lot more!
Is it a manual or automatic 'change-over'?
Are you trying to start it on gas, and run from cold on the stuff?
Are you starting on petrol and imedietly switching to LPG when running, or are you actually waiting until the engine is possitively up to temp on petrol before switch-over (heat on windscreen test?)
Does the system 'force change', when you blip the throttle, and is switching to gas before the engine is up to heat without you having any contiouse input to the process?
Thats the first lot of things to consider, and the most likely cause of using so much fuel.
You NEED to make sure the evaporator is at temperature before change-over, test by touch, if you cant use the demist from the heater, or get one of those remote electronic thermometers from maplin and put the temp sensor on the evaporator and HOLD the change until its reading 70+ Deg C
Next is the gas getting to the engine? Look for leaks, most likely downstream of the switch over solenoid, or the evaporator.
Who installed the system? When? What valves / fittings / pipe work did they use?
Next, mixture settings; delivery pressure regulator on the evaporator, idle speed screw and mixture setting screw.
Should be running with the least evaporator pressure possible to meet the engines maximum damand on load; its often wound open too far and choked at the idle screw, giving too high a delivery rate, and a ritch mix on part throttle, but better pick-up on acceleration....... but excerss in volume you describe is ALL the screws fully open!