steve you might not like this idea but, have you considered taking the entire unit out, and swapping it for an aftermarket alarm/immob/central locking?
Or if you're broke like me, buy a halfords/maplin ?30 central locking/remote switching device with keyfob..
You might find small garages have difficulty in taking out std alarms as they might need wiring diagrams / tech notes that only rover have. (on the other hand if you are feeling adventurous you could re-wire it so you have a total alarm off switch (i know my pug306 used to have a switch on the column so if you left a dog or small child in the car it wouldnt go off)
its what im cosidering atm mate, rhinopower might be sorting e out with a low-hassle clifford.
i normally would have a fiddle myself but their both cat 1, with sensors everywhere, and i think it starts beeping at me at the slightest thing, not something i can live with!
cheers anyways matey