After Market Alarm/Immobiliser at Fault?

Hello, I've got an aftermarket Cobra Alarm/Immobiliser fitted to my 99 Octy 1.9Tdi Estate (was on when I brought the car). Normally if the immobiliser is on the LED flashes, and if you try and start the car all the warning lights, except the glow-plugs, come on and the engine just cranks. Switch it off with the igntion on and the glow-plugs lights come on, and the engine will crank and fire.

About a week ago (having not used the car for a week) the immobiliser part of it would not switch off, engine would turn but not catch. It still wouldn't turn off using the overide key fob, or the igniton on/off sequence. Anyway after some more pressing of buttons and rude words it switched off, engine started and I was a happy bunny once more.

Until today that is, was driving along the A46 (three lane dual carriage-way) when I noticed that the immobiliser LED suddenly lit up, then the engine died and the LED started to flash as if in a normal on state...... Not fun having the engine died doing 60+... Anyway, managed to pull it off the road and up a slip road - but it didn't want to start again. This time as last time the car immobilisition warning light goes out - so I know that the car is happy with the key.

I've checked the alarm fob - batteries are fine, car is in good condition - had major service 3k ago, car battery has good voltage..

Anyother thoughts before I remove the stupid thing - btw its not the first time I've had trouble with this alarm - about 6 months ago if I set the alarm active, it would flatten the battery over 48hrs, if I didn't set the alarm the battery was fine. And leaving the battery disconnected from the car - the battery held it's voltage...

Oh The Alarm/Immobiliser in question is a 7928 with a 1984 immobiliser