98 3.0 Alarm gone mad? Indicators flickering - quadmyre

After a spell parked up inactive in wet weather (oh I love french cars) my 406 has developed this fault...

When the ignition is turned on all seems well for a minute or so then it looks as though the alarm goes mad. The siren thing is disabled anyway but the headlights come on and the indicators flicker like mad (can hear a relay clicking away in glove box I think). Starting / stopping engine, locking / unlocking doors with keyfob seems to have no effect. The only thing I've found that cures it is either disconnecting and reconnecting the battery (which stops it temporarily) or pulling fuse 15 behind the immobiliser keypad (the one for the alarm).

Any thoughts?

and / or

Is there any detrimental effect from running the car with this fuse out? Seems to kill the seat position memory too but that's about it.