[jackthehat.co.uk]

The first thing I would do is the standard checks to try and pin down the battery drain problem (if there is one).

Get a digital multimeter and connect it to the car battery so it gives you a continuous readout of the voltage. Make sure the car doors are all shut, the keys are out of the ignition and everything is switched off. Now open the fuse box lid and remove each fuse in turn whilst looking at the voltage reading. If you remove a fuse and the voltage suddenly leaps upwards by a volt or two then you have probably found the circuit which is draining the battery.

I had this problem on my old Granada and it turned out to be one of the headlamp wipers. It was out of position and was constantly trying to reset itself. This was flattening the battery in less than two days at a time. I solved it by leaving the fuse out and hacksawing off the wipers ;-)