It strikes me that what you want is about for the kind of money that you are talking about buying a petrol for & converting it to LPG
Like this http://www.pistonheads.com/sales/2454900.htm
Looking @ what you are considering spending to get yourself a petrol + the LPG coversion £5150 ish this with a little haggle maybe £5700 or less if your happy to drop the warrenty maybe. Then the on going cost of fuel (not likely to be comeing down) the difference in cost right now will seem like peanuts in three or four years
Id've thought 

Like this http://www.pistonheads.com/sales/2454900.htm

Looking @ what you are considering spending to get yourself a petrol + the LPG coversion £5150 ish this with a little haggle maybe £5700 or less if your happy to drop the warrenty maybe. Then the on going cost of fuel (not likely to be comeing down) the difference in cost right now will seem like peanuts in three or four years

