Ollie, you haven't changed the central locking/alarm box have you - behind the drivers side kick panel??
Or if your one is fooked it could be that.
My boot popper stopped working and kept draining the battery and I traced it to the central locking box :-
looks like this
http://cgi.ebay.co.uk/Escort-mark-5-...#ht_500wt_1182
I tried fitting one from a later car (blue label) and it did the exact thing you described!!!