Deactivating my Altex Alarm?

I have recently moved into a house with an Altex alarm. The previous owner did not leave a manual but said enter the 4 digit pin to enable the alarm, and enter it again to disable it.
This does not work!
I seem to have to enter the pin, wait, the alarm starts beeping - loudly, i press a few buttons in a panic and then have to enter the pin again to stop it...
What do I actually need to do!?