As an update, I have replaced the earth strap, which had snapped off completely.

Took the car to where I brought the battery, they tested it and said alternator is ok and battery is also fine.

Since the car has been playing up I have removed the bulb from the boot light and switched off the interior light so that it can't come on.

I've also checked the wiring on the stereo as it has had a different one put in, whoever put it in used the proper loom and all looks fine.

My girlfriend, who uses the car the most, told me today that a dash light has come on a couple of times recently.

Having slapped her a few times I have managed to discover that it's the door ajar warning light.

So it's probably a dodgy micro switch somewhere? The alarm hasn't been false alarming and all interior lights are disabled so would this be the cause of my possible battery drain?