Usually people ask that question if they are having to factor in the cost of the conversion, so low mileage means it takes longer before they are into a saving. However, if it is already fitted then that isn't relevant so which would you prefer to pay, £1.40 a litre for fuel or 60p a litre for fuel? 2,000 miles a year would cost you roughly £800 on petrol compared with £350 on LPG so you are likely to do more miles as the cost is less of an issue. I did an 800 mile round trip the other weekend and it cost me less than £100 in fuel. Not to mention that LPG is a much simpler hydrocarbon fuel with no noxious additives so not only are you causing less pollution, the oil stays clean so lubricates better and your engine will last longer.
I've run on nothing but LPG for the last 12 years and would not consider running anything else. Even when considering a different car (as an addition rather than instead of the P38 of course) for me the first consideration is can it be converted? With running costs less, or at least very similar to, an equivalent diesel but with none of the downsides. My car is registered with and exempt from charges for low emission zones in France, Germany, Belgium and Holland purely because it is running on LPG.