A couple of years ago, I was suffering mystery starter battery drain problems. I bought a new battery, but that didn't cure it. As has been mentioned, I think it's a combination of radio and immobiliser that cause the problem.
I had the big yellow van out a couple of times, and they carry starter packs, which are simple to attach to your dead battery with the croc clips.
So, I thought, let's cut out the middle man, and I bought a starter pack for myself. I also bought a battery isolator for when I leave the van for any length of time.
Of course, this is a bit belt and braces, but I have never had a flat starter battery since.
The battery pack also has a flash lamp, hazard warning lights, and spare 12v power supply, so is a useful power supply for camping too.
I notice you can also get one with an air compressor combined. That would be useful.
These are some examples:
http://www.ebay.co.uk/itm/like/33168563 ... rmvSB=true
1990 Autosleeper Trident. Water cooled 1.9L DG + 4 speed manual box