Normally I press the fob and one flash of the indicators confirms the alarm is set. To unlock, it's two flashes.

Recently I've been getting 3 flashes when locking and within 5 minutes the alarm goes off.

Three flashes indicates a "door" issue rather than bonnet or boot - does this mean a problem with the door locks, or that there is a sensor somewhere than needs repairing?

Central locking with the key still operates fine.

We had a Toad on both our Bongos and they were very reliable.

As you say it indicates a fault with a sensor or that something is not closed properly. Is the bonnet alarm 'plunger' fully depressing when closed?

Could the central locking be out of sync with the alarm fob? Ie the toad thinks doors are unlocked.

Or one of the sensor switches is faulty.

Worth eliminating the obvious first...

Thanks Toby, do you know where the other sensor switches are besides the bonnet and boot? Also there are two internal movement sensors by the A pillars.....

Realised after all this time there are two microswitches, one above the other, in the door jamb. I found the upper one in the passenger side showed resistance until I pushed it fully in - this appears to have sorted the fault

Sorry for the delay in reply.

Glad it's sorted. Result !

Yep thanks for the pointer Toby, it's been fine since!