Veritas 8 Cover Tamper alarm overide?

I have moved into a flat with a veritas 8 alarm box and no instructions. The box is buzzing loudly, but if the box is held tight the buzzing disappears - probably just 50 hz hum resonating from a power transformer. I have done a temporary fix by jamming a wedge between the box and the wall. I need to get the cover off and tighten the mounting screws without setting the tamper alarm off. I assume I will have to enter an engineering code and then an override code to get 'quiet' access to the box internals. Then a reactivation code to reset the box tamper alarm to normal. Hope someone can help.