How can I fix my Accenta 8 alarm system?

We moved in to our house in October of last year and there was an alarm system installed (Accenta 8). The previous owner said they didn't use it much and left us the code. We have never touched it or set it since and just about a month ago, the alarm went off about 5am in the morning but it hadn't been set. It was that loud my boyfrien disconnected the main sound box under the staris by ripping the wire, then all that could be heard was the sound from the keypad which eventually went off itself cos we had misplaced the code. The outside alarm also went off. After then finding the code ever since the alarm has been sounding when it feels like it!!! We don't know why, the outside one now thankfully only goes for about 3 secs then the keypad starts! It's driving us mad, we have removed the battery incase it was needing replace but yet it is still doing it. Can anyone help?!