Please can any one help with accenta optima alarm code not known!?

We have recenyly purchased a new property which has been fitted with an optima g3 alarm unfortunatlywe are unable to get hold of the alarm code as the property was a repossion and the previous owners unknown when switching sysytem on the siren sounds and key pad lights tamper etc illuminated. i have recharged the battery in the alarm unit and have tried using the factory code but no luck.