To try to identify which circuit is drawing current then you will need a multimeter that has at least a 10Amp current range - do you have or can you acquire such a meter (
Maplin do some cheap suitable meters)?
If so using your meter set it to the 10Amp current range, to start with (you might have to move the leads between sockets on the front in some meters) now disconnect the starter battery +ve and place the meter between the +ve terminal on the battery and the +ve connector. It should read a very low value well less than 1 Amp, most likely just a few tens of milliamps. If it is a very low value then that is good, if its more than 100 milliamps then you need to identify what is drawing the current, so start pulling fuses and check if the current drops. When you have identified what is drawing current or at least the fuse number - report back here! If none of the fuses cause the current to drop then you will need the help of an auto electrician !
If the current is only a few milliamps, then it isn't something drawing current - are you charging the battery sufficiently during runs out ? Some one recently, during the cold weather, only popped around the corner a couple of times a day and had the same sort of problem, the battery just wasn't being charged enough.