Running LPG converted cars off central heating gas?

Possible. CNG (compressed Natural Gas). Extremely difficult to compress enough gas to go mor than 100 metres.

Volvo do one, but therearen't many filling stations.

http://uk.cars.yahoo.com/15082006/49/volvo-s60-bi-...

If you mean bulk LPG from a tank, yes it's the same stuff. Some people use a hand pump, some an electric pump. The main problem is that it isn't intended for road use, and hence hasn't had duty paid. I believe you can arrange to self-declare what you have and pay what is due. If you don't do so then it's as illegal as using red diesel, and presumably the same penalties, eg. a stiff fine and confiscation of the vehicle. I don't know how easy it is for them to detect it, perhaps they can monitor the volumes delivered to individual properties, or pay bounties to delivery drivers who spot vehicle filling equipment.

LPG and CNG (compressed natural gas) are different things and need different conversions.

If your car is LPG converted you can run it off bunkered household LPG (usually if you live out in the boonies and can't get normal mains gas). IIRC bunkered LPG comes in at 7p per litre or something ridiculous. You can get a pump installed but officially you have to declare and pay duty on the fuel you use for your car.

It is possible to get a CNG conversion for your car, as a previous poster said Volvo did a bifuel CNG car for a while. AIUI the trouble is that there isn't enough pressure in the household supply so it takes overnight to fill up from your mains gas supply.