Thank you for your replies. Yes, the 3 wires from the panel was a puzzle to me too but I was convinced the alarm could be switched off remotely when activated - guess I was wrong !
I can only recall the alarm going off 2x in the 22 years it's been fitted. One time I wasn't far away from home and by the time I returned home the alarm & siren had stopped so maybe that the panel was set to sound the alarm only for a short pre-determined period making me think the remote disablement had worked.
A follow up question - Are there alarm panels available today which can enable/disable the alarm remotely if required ? If so, any recommendations!