I would try and trace the cause of the battery draining first of all. The easiest thing to do here is open the boot, find the battery in the top right behind the carpet, remove the negative terminal and put a multimeter in series with the earth cable and the negative terminal of the battery setting it to read current.
Make sure you don't crank the engine with it setup like this as you'll damage you're multimeter - keep the keys out of the ignition. You need to then measure the current being drawn. This should be lower than around 150mA (anyone else?). If it's much higher than this, then you have a problem with the electrics, if it's lower than this then your battery is probably knackered.
If the current is much higher, go through the fuse box in the drivers footwell. Remove each fuse and see and if the current drops (put the fuse back in before moving to the next). You really need two people for this job, one to shout out the numbers, and one to pull out and replace the fuses. Usually you'll find one fuse where when you pull out the current drops significantly, and that's your faulty circuit. I've had one where the aerial had burnt out in such a way that it was always drawing current and draining the battery.