£900 sounds about right
You gotta do quite a lot of miles to warrant the conversion cost.
LPG is about 40p per litre at the moment, and apparently the MPG for LPG is worse than petrol.
I was considering getting it done. but thought against it, we only do about 5-6000 miles per year so it's not worth it.
The LPG tanks are normally put in the spare wheel well so you would have to put the spare in the boot !.
LPG is Liquid Petroleum Gas, or some places call it autogas.