I bloody love all these people running to the defence of LPG motors. If it's that epic, why isn't your car LPG'd? (exception of Entwood of course). 
My car is LPG, and on a long run, it is very, VERY cost effective. Short runs, just added weight to a standard car, extra money for servicing.
My argument is simple..... a cheap to run car like a 1.2 Corsa, the savings will be minimal. A thirsty 5.0 Merc, it's a no brainer.
If I ever own another Omega, it'll be (as always have been) a V6. The first thing I'll do it LPG it. I'll LPG is because it'll save me a lot of money. If I were to buy a 1.2 Corsa, 1.6 Astra, even a 2.0 Vectra, unless I had the kit in the garage, I wouldn't bother because it would be a waste of money. I wouldn't necessarily turn down the aformentioned cars if they had already been converted, but I very much doubt the tank would see gas.
Eh? Both my cars are LPG'd.

Do not understand your stance about savings. The savings are primarily based on the price of a cheaper fuel. If you had an Omega and a 1.0 Corsa and both did 40k, the you would achieve a saving on both of them. Of course the saving would be greater on the Omega, purely because you just used more fuel to run a heavier car. But you would still save on the lighter smaller engined car. If you annual mileage was less than 8-10k, then I suspect even the omega would not break even at the end of year 1 (based on a 10p/mile saving and costs as below).
Sure you have to work out savings and your annual mileage before installing a new system, but on a pre converted car, it is a no brainer, especially if you have some LPG knowledge and are willing to check the installation is of a good quality. And why would you not fill up the LPG tank? Makes no sense at all. Unless you only do short trips, in which case LPG is not the fuel of choice, no matter what the car size.
In addition remember a 4 pot conversion of good quality can be had for £750 - 800. A V6 conversion like mine was £1200. Entwood's was double that iirc?