Alarm sounds when locking car.

Before anyone says " do a search", I did,
Right. 1995 Disco TDi. Problem 2, ( it will bcome clear!) sometimes when I lock the car with the fob ( two button fob) the alarm sounds straight away. I can lock the car with the key, sometimes the alarm will sound as soon as it locks, usually its ok, but just now, it locked with the key and was silent, but I noticed the flashers all flashing.
I unlocked with the fob and relocked it with the key and its all quiet now.

This only just started happening today, but over the last couple of days I noticed problem 1, ( see?) if I stop the car and sit in it for a while, the key will not operate the starter unless I press the unlock button on the fob. I never had to do this before, I assume the immobiliser is operating and I need to remobilise it. I had to do this in a Golf we had but that was everytime.

Any ideas?

Allan

Is there a faulty door closed sensor? My D2 beeps the horn if there's a door ajar when I try to lock it with the button.

Hi Allan. Didn't so your disco at this years Whitstabubble festival only mine in the big car park

Re the fob bit that's correct as the system normally expects the car to be unlocked and the key inserted and the engine started straight away, after a few minutes the key symbol will appear, so press the button or move the fob nearer the starter switch.

Re the alarm if all's well when locked by the key that would suggest that the ultra-sonic movement detector has a fault (over sensitive) so just but masking over the send-receive elements or even unplug it then lock with the fob and see what happens next.