mmk...
i dont have a brant so i'm not sure on specifics... but i'd say your 'receptor' or whatever its called (the thing that picks up the infrared) may be buggered. bit strange how you have a stock system and this happens...
i lost my immobiliser off my keys once... had to get the car towed (set off the alarm a million times) and when i got home i took out alarm system, replaced with a mongoose...