So how long should a battery last if the car is left for long periods? Mine would not start after about 3 weeks last winter, but in that time I had had the doors open cleaning it one day and fitted the new RCD510, so was playing with that too. Even so, I wouldn't have thought that was a great deal of battery usage. My Mk2 would have let me done all that.
3 weeks in winter is a fairly long time to leave a car, and there's a number of factors to consider; how much charge had the battery got before you stopped using the car? Do you do a lot of short trips [especially in winter] when everything electrical is on most of the time? The battery is never fully charged just from the alternator unless you've been on a long run. Also just simply charging it when it's flat won't do it any good in the long run, you really need to condition it as well.
There are several items in the car using power continuously; alarm system, RCD/RNS, ECU etc, they all take their toll especially in cold weather.
If you garage your car for part of the winter you really need to get the item I suggested in my first reply [About £50 for complete peace of mind - bargain]