Really could be anything mate.

Easiest way to check how much current is being drained is to disconnect a battery terminal and connect in an ammeter between the battery and terminal.

Lock the car up as you would normally (if you have an underbonnet switch for the alarm, this will need to be held down or the alarm may not set right and give an iffy reading), look at the reading on the ammeter, it should read under 0.1 ideally. If it's any higher, then start pulling fuses one by one (putting them back after!!) and see if the current drops. If it does, that's your faulty circuit.

If pulling fuses makes no difference, try disconnecting the alternator, starter motor etc...

Where abouts in Hertfordshire are you? I'm in Watford, so if you're not a million miles away I don't mind having a look at it for you if you want?