Vertias 8 alarm system. how to change the bell cut out time?

hi
I have a veritas 8 alarm, which has been installed for 10 years now. The alarm went off on friday, just as we arrived on holiday, we recieved upteen complaints via telephone and upon our arrival back, that the bell was sounding too long. How do I change the bell cut out time to a shorter time. The alarm company that installed it are no longer in business and I have tried the 1234 & 4321 engineers codes to access the programming menu to do myself but they have obviously changed the default to one of their own. How do I access the menu without this engineer code, or over ride it with another.

This is the first and only time my alarm has sounded, but I cannot cope with all the complaints that the bell sounded for too long.