Car Alarm?

Hi all
I have just acquired a P reg rover 216 cab.Its a great looking car.But my problem is when I lock it up,the alarm keeps going off for no reason.The alarm system its self is factory issue.Is there anyway I can immobilse the alarm system and not effect the car immobiliser its self.Or are they all one system.Or does anyone out there no why its doing it.So I can take it in and not get ripped off with bulls...t science and a large bill from an auto electrician.Hope you can help.Thanks for reading.