Mine always starts on petrol and changes over to LPG once it warms up a tad. It has a 25L Petrol Tank and 55L LPG Tank. I just top the petrol up when the Low Petrol Warning Light on the dash illuminates.
It does tend however to run rough on the petrol when cold and seems to misfire but once it switches over to LPG away it goes like a train.
I wonder why some lads call LPG the Devils fuel when it only costs 69p a litre and saves a fortune.