I converted my 2000 (X) Picasso 1.8 petrol to LPG at a cost of £1500, I figured that it would cost me 2 to 2.5 years before I got my money back. I joined "Countrywide" Lpg as an account holder and never regretted it. In September to December 2005 petrol was an adv of 93p a litre and I saved £281.70 by going Lpg at 38p. in 2006 I saved £970.28 at an adv of petrol 97p a litre and gas at 43p in the 12 months, and so far this year to Sept it has saved me £844.94 at petrol 96p per litre and gas at 46p. It has paid for itself in less than 2.5 years.
The only problem I had was in 2006 when we went to the south of France and did not get the car to run on LPG in hot weather, as soon as we got back to the UK it started to use gas again and my garage said next time to let them know and they would calibrate the car. Meanwhile no problems.
LPG is paying for my full service, tax and running cost during the year as well as some of the wifes car. Convert to gas or buy a certified lpg converted car and save the cost of running it for the year. The chanceler will increase the price of gas but untill then get the benefit.