tannera - good post and some interesting stuff in it.
Can I clarify how you have 'saved a fortune'
Based upon your numbers of an £1,800 conversion and LPG at your average of 70p. I we say petrol is £1.40 then the saving is around 70p per litre. Turn that to gallons is £3.17 per gallon. If we go on 30mpg then the cost of the conversion equates to 567 gallons and 17,000 miles. Add back the 20% loss in MPG from LPG and gets me give or take at 20,000 miles to break even.
Frankly that represents 3 or 4 years motoring for most people in a Zed, so unless you have put that sort of mileage on then you are still well out of pocket. I don't doubt if the combination of high mileage and long ownership you will break even.
As I've posted before I'm in no way against LPG on the right application, but for me the numbers don't stack up unless you're a high mileage user or a really low mpg car.