Moss MS2000 erratic alarm?

Answers - mend, repair, fix

Hi Noticed this old thread
I am having a similar problem to the one in an older thread (2008). The alarm has sudenly started sounding erratically. Unlike the old post, I've never used batteries in the unit and don't use the external siren. I've tried unplugging for a while and tried another adapter but no change. It's worked great for years but sadly suddenly acting erratically - and typically middle of the night!
Any suggestions as a I really want to keep the system.
Cheers
Peter