Yeah, I think as it cost me £75 to tank it & only got 320 miles odd it defo needs LPG'ing 
So assuming petrol is 129.9, which is what I paid last night - you would have got about 57 litres for that money - so, 12.7 gallons.
Therefore, you've got 25.2 miles per gallon, which is 23.45p a mile.
On a V6 LPG, you can safely assume you'll get 15% less economy, so
21.42mpg on gas.
Basing gas on what I pay here, it's 76.9p per litre. On that basis,
you're likely running cost on gas comes out at 16.33p per mileTherefore,
your saving, running on LPG, works out to be 7.12p per mileYou've said you do about 4k miles per year (from memory?) so therefore
by running on LPG, you'll save £284 a year.Also, based on the above savings per mile, if you buy an LPG kit at around £800, for argument sake, (which it easily will be by the time you've bought curry and beer

)
your mileage to break even, will be 11,111 milesYour kit will take over 2.7 years to pay for itself.
Is it really worth it?
On top of all that, if you're doing lots of short trips, you'll be initially running on petrol for that anyway, making the cost savings even less.
Hope this gives you some objective figures to consider / work with, in making your decision
