Veritas 8 alarm has gone crazy!?

The alarm was installed by an electrican approx 5 years ago. The other night it was part-set and I was asleep. It went off for no reason. I entered the code, pressed reset and re-set it. Needless to say as soon as I'd fallen asleep it started ticking as if someone had entered the front door. I ran downstairs, the alarm went off, I entered the code, pressed reset and left it off. It then started making DA DAAA, DO DAAA noises (like error chimes) for the rest of the night. The next day I noticed it was making chime noises when you opened the front door even though it was off! My Dad has tried to change the battery today (which I know is a tamper to the alarm) and I've just been told it's going off again and entering the alarm code won't stop it. HELP!