Also here is something the guys at RRSport.co.uk (range rover sport fellas) said about LPG conversion:

• LPG conversions can be done on petrol engine cars (of course!) and also diesel but rarely.
  Conversions costs vary from about £1700 for a small family car upwards
  JE Engineering will do it for £3K for a 4.4 petrol, or £3500 + VAT for a Supercharged FFRR, RRS SC or D3.
  The Supercharged engine, the Sport and the Disco conversions have been problematic and have engine fault light issues - JE are suspending conversions until this is fixed
  You'd need to spend a large amount on Petrol or diesel to recoup the costs ( I do a minimum of 20K a year and would still need to achieve 16 mpg from a FFRRSC to pay it back within 2 years!)
  Doing 20,000 miles a year yould save £1600 on fuel as compared to diesel if you manage 16mpg.
  There is a minor reduction in your Tax disc for getting it done (£10-20!)
  Your insurance WILL increase marginally
  Any Land Rover warranty on the car is completely and TOTALLY voided. Even if the stereo breaks, you cannot claim on warranty, they will not touch your car.
  You will also have to take your car elsewhere for servicing for the same reason
  Your MPG will decrease on LPG by about 2-3 miles and the performace is marginally affected also
  The LPG tanks although stated as 95 litres can actually only get about 85-88 litres filled and sit in the spare wheel well
  You keep the standard fuel tank so you effectively have nearly 200 litres of fuel on board
  The engine always starts on petrol then switches to LPG automatically
  A small interior switch tells you the amount of LPG left
  The conversion takes about 5 days and typically comes with a 2 year warranty