Let me clear up a few points here as I have LPG on my Clio 172.
It does add a bit of weight, but not massively, unless you run with full petrol as well as LPG tanks. The LPG tank is made to a much higher standard than petrol tanks as it needs to hold a pressurissed fuel, so is heavier
The government no longer give a grant.
It should cost between £1250 - £2000 for a clio
I cannot notice the difference in power, most modern LPG systems are multi point injection systems. I have tried switching between petrol and gas and cannot notice the difference, although I had it tested on a rolling road and it gave 4 bhp less on gas
You can have a tank that fits in the wheel well - as I have - that saves boot space, only trouble is spare wheel has to sit in the boot.
The government are reducing the subsidy on LPG, but only at 1p per year more than petrol. I pay between 50-54p per litre. I get about 25% less MPG (20% is common as LPG has about 15% less energy than petrol )
Gordon D is correct, about LPG, but I think that the multi point sequential LPG system is more common now, LPG octane is as much as 110 RON.
There are many factors to consider when choosing LPG, one of the most important is the scarcity of LPG stations and the variation in price. Make sure that it available near you at a reasonable price - half the price of petrol or less.
Despite rumours to the contrary, LPG is very difficult to light and the ignition system has to be top spec to ensure no misfires.
I do about 650 miles a week and save about £25 per week, so my system took about 1 year to pay for itself. Am I glad I converted - yes. Is it for everyone - no
Main advantages are it's cheap.
Disadvantages are it's more difficult to find, slower to fill up, reduced range and potentially reduced boot space.
Choose installer carefully, there are plenty of cowboy LPG fitters
Graham