Hi Mark - I will also post this on Technical.
The Grants for LPG conversion are through Powershift - a search on Google or similar should give you lots of info including certified installers. My understanding is that only vehicles up to three years old are elligible. I have also been told that grants have "dried up" of late due to lack of funds/lots of people taking advantage of the scheme. Have a look at www.lpga.co.uk
The Tank(s) Four options
a)- On a Land Rover there are lots of places to put a tank. I had mine positioned down the side of the Range Rover Boot so that the load space with the seats down was still there. Some conversions place the tank accross the floor behind the rear seats which I did not want.
b) Another option is to have two long thin tanks slung underneath the floor on either side, strapped to the chassis. This is good as it leaves the interior unaffected but does limit what you can do "Off-Road".
c) The third option is to have the petrol tank removed and replaced with a LPG tank(s) then have a smaller petrol tank fitted elswhere - after all if you can run on LPG at c 35p/L the petrol tank becomes virtualy redundant.
d) Fourth option is to take out the spare wheel and have a "donut" LPG tank fitted. You then need to either carry a spare somewhere else of have your tyres treated with that "goo" that stops them going down even if punctured.
LPG Outlets - You can get a list of filling stations from the LPG Association website. There are far more out there than you would think. Calor Gas centres on industrial estates are often good places to buy so LPG is not just available from petrol stations. If you are going somewhere on holiday we print off the list for the towns on route/nearby then use www.streetmap.co.uk to print of maps of where they are. Never had a problem getting LPG - just take a little planning.
Converstion - Again a list of LPG Association authorised installers is available from the LPGA website.
Other considerations? Make sure you are happy with the vehicle youconvert - if you pay