I spent a great deal of time looking into LPG, quotes, reading up etc.... you would with 18mpg!
Anyhow, i found alot of people who claimed their cars were unreliable and the LPG systems were nothing but problems. One chaps my brother in law who ran a fleet of vans on LPG, I know not everycar will have problems. There are those on here have had nothing but bliss, but from my point of view talking to so many who were unhappy it's not worth the risk.
It's also supposed to give you 80 -90% of petrol consumption not 100% One guy i spoke to with a rover 75 got around 25mpg on petrol and only 35 on gas, hardly double. Don't forget thats actually 18mpg at half price! not a bad saving but if your paying £1400 for the conversion it's not so appealing
Thats also way to expensive, i was quoted around £1300 - 1500 for my V6. 4 cylinders are more like £1200 if you check out traders on ebay.