[kaiser]

I don't understand the pence per mile. You should measure in liters or gallons.
Lpg has got a higher calorific value per kg, but is far less dense. Depending on blend, you will always use up to 15 to20 % more volume (liters) of lpg, compared to petrol.
Since lpg is cheaper, it cost less, but your consumption in volume is higher.
That has nothing to do with your injection.
It is simply a function of how much energy you have per gallon.
There is something fundamentally wrong with your calculations.!