After reading up on the boards about the alarm system, everyone seems to say the alarm and immobiliser/ignition are seperate unlinked entities, but last night I found out this is not true.
I noticed for a while when unlocking and locking with the remote, the bleeps stopped coming, but central locking still worked perfectly.
Last night, I decided to unlock the car (no noise as usual), then turned the key so I could switch the headlights on as I was trying to find something. As soon as I turned the key enough to get the lights on, I heard a bleep that was the alarm activating, then it set itself off!
After closing the door and locking/unlocking a couple of times, the alarm started working perfectly again. I didn't dare turn the ignition to see whether the engine turned as the timing belt is off (and I'm guessing the immobiliser probably cuts sparks or fuel rather than starter anyway), but this definitely shows there is some link between ignition/immobiliser and the alarm system.
Car is a 98 v6.
Just thought I would post this, as it maybe useful to someone.