battery draining with in 2-3 days

Hello...the battery drains within a few days on my J-reg RRC if I'm not using it. I'm now iffy about using it in case I don't get from where I'm going! Other sites claim the immobilier will do this (its aftermarket) but I've also heard that these take virtually no power...? Is there a simpleton's way to check alternator etc or will it be the case of letting the auto-electrician loose on it?!

If you have got a multimeter put it across the battery terminals with the engine running, it should be charging at 13+ volts. A lot of auto electrical specialists will check the battery for free to see if it's faulty.
Not sure if it would be the same or not, but I had similar problem with my disco, turned out to be a stuck accessory relay. Are any of the electrics still live with the ignition off? Radio, seats etc..as that would point to a faulty relay.
You could try disconnecting the battery for a few days to see if it still discharges, which would rule out the battery

Extracts from tech pages:
Battery (with everything off) should read more than 12.5v, crank over & batt should hold to 11.5v. A good reading with engine running @ fast idle (alternator output) would be 14.4v. - To check for 'parasitic' drain disconnect battery earth terminal & check reading between that & battery earth post, should be below 0.05A Hope this helps.