Have you tried putting a voltmeter on the battery when the car is turned off and when the alarm is switched off and switched on to see if its draining the battery. May turn out to be your alternator, I had the same problem a couple of years ago on my Rs Turbo, it was charging fine when running but once it was switched off the alternator was draining all power out of the battery!

If not have you checked the likes of your boot light isnt staying on or the cd player (if not standard) has been wired straight to the battery rather than the ignition.

Not sure if this is any help but thought I would sugget anyway.