The 918U (U = ugrade) alarm turns the Thatcham Category 2 CODE immobiliser into Thatcham Category 1, but all it does is add remote central locking with voltage and interior movement sensors, plus the alarm siren and connection to the indicators, there's no connection to the CODE system although if fitted correctly the power to the starter motor solenoid is supposed to be re-routed to come from the alarm ECU, but sometimes this hasn't been done so you can quite happily start the engine and drive off with the alarm going off.

If it is goosed, you might be able to pick up a new-old-stock unit on eBay (I did!) but I would also take a look at the siren, take it out and open it up and make sure there's no water in there, also check any in-line connectors int he wiring.

Do you get any flashes from the LED if you connect that?

The right hand button on the remote is a "panic" button that will immediately set off the alarm if pressed, perhaps the contacts under the button are dirty or the button is stuck? (you did say it was a bit minging inside)

Do you have another remote fob (system should come with two)?

Have you checked the dip switch settings on the alarm ECU?

Have you checked where the little white 4 pin connector goes to? I can't remember where it goes but I think on mine it goes to some further little connectors and there may have been some fuses there too.

But at the end of the day, no matter how attached you might be to it, it's only an alarm, and can easily be replaced by another one.