A few people on here have mentioned it but, as far as I'm aware, no-one has actually converted one yet.
The problem is that it doesn't seem to be very economic. I had a spreadsheet which calculated the costs, the savings and the time to re-pay the conversion. It seemed that it would take around 5 years to pay for the conversion and, after that, perhaps a saving of £300/yr. So, after 6 years, you'd be £300 up
In addition, you lose boot space and the consumption of gas is considerably higher than for petrol/diesel. Then, just to top it off, the only reason LPG is cheaper than the other fuels, is that the government doesn't tax it so highly. When they do, it will rise up in price, much in the way that diesel has.
So, how does it really make sense? In my view, only if you buy a car with the conversion already done, in which case you could save a few hundred quid a year, at least for now. Would be interesting to hear your thoughts, since you already run a couple of LPG cars.