On the plus side, I now have a job that is only 9 miles from home, rather than doing 500 miles a week..... so fuel economy is not such an issue anymore... meaning I could potentially have something mega beastly.
A 3.2 springs to mind then 
Forget LPG'ing it.....you'll never get your money back doing 18 miles a day plus the odd run
It is paying off on mine saving £80 a month on 12 miles each way, with heavier savings on long runs.
This would pay a kit off in 12 - 18 months, especially if James keeps doing long trips
I look at it like this Martin.....if you buy a thirsty petrol.....maybe 15-20mpg av as appose to a more economical diesel...poss 40mpg av.....do about 40k miles a year......diy it yourself then yes i agree....12 months is possible.
Payback time on mine using the above figures (i payed £1800 to have it converted) is around 18 months.
Im saving about £100/month on fuel in my omega compared to the merc diesel i used to use, doing about 40k a year......so i have to look at it this way