ade compact optima?

my alarm and outside bell was installed around a year ago.the engineer told me that i needed to change the battery as it was rubbish.
i did not change it though and the alarm was working fine.
a few weeks ago the alarm switched on in the middle of the night and it has been doing this on and off.
i decided to change the battery last week.But the alarm still swithes on at night.roughly 1 of 3 night it will play up around 2-3 am.
i use only 2 zones,zone1 covers all entry points downstairs and zone 2 is for a single PIR upstairs.
sorry for lengthy post.
your help is appreciated
thanks in advance
ed