Before you purchase an LPG vehicle look carefully at the date it was first registered. Yes, it's true that LPG vehicles are MUCH cheaper to tax than petrol and diesel vehicles BUT ONLY IF FIRST REGISTERED AFTER 1ST MARCH 2001. I own a 1.6 dual-fuel Astra first registered 5 weeks before that date and it costs exactly the same as a petrol or diesel model which is £185 per annum.
Also even though your LPG vehicle will allow you to travel in the London Congestion Charging zone without paying the daily charge you have to register your car first which costs £10 AND send in a certificate of conversion. You can obtain this (or at least I did) from the manufacturer if you own an Astra which was converted by Vauxhall, for aftermarket conversions you have to get this from the company which converted the vehicle.
That said, I'm THRILLED with my LPG car, especially when I refuel (and there are many petrol stations throughout the UK who stock LPG - finding one has never been a problem, you can even download LPG Forecourt POI files for your SatNav).
I thoroughly recommend owning an LPG vehicle and when I have to change my car I'll be looking for another dual-fuel Astra.