I have a single point system on my p38, leonardo i think.
if you have the rocker switch with lpg on one side and petrol on the other,
It can be started on lpg, with ignition on before starting, flick the switch form gas to petrol and quickly back to gas, the gas led should stay on and then start the car.
the car should run, but you may find the vapourisor will freeze, especially in the cold weather at the moment.
I have been thinking the same for my car, not that i do short journey's, just for a quicker warm up.
will see if any one else can help.
I don't konow if Gavbriggs has a preheater on his, he has something written after his postings