No one seems to know anything about alarms here so no one ever replys (i posted a similar question and got no replys).
Anyway, after reading some threads on the alfa156.net forums, it seems that the alarm (which is located under the passenger side front wing, take the plastic liner out and all should be revealed). has it's own battery. Disconnect the whole unit, then remove the battery. Leave it for 10 mins, then plug the battery in. Attach to the car and go for a long drive. What your doing here then is reseting the alarm unit and then re-charging the battery.
If that doesn't work, take the battery out of the alarm unit itself and measure the voltage. I am not sure what it should be but it should be printed on it. If you get a 0 voltage then the battery is gone.
I was lucky in that my alarm problem was simply a case of the car battery causing voltage drops. It set the alarm off and would be unset by the key (tamper alarm i suspect).
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