Nexus 8 Wired Alarm

[Ruth]

Hi All, 
I'm new to this so I'm not sure if I'm in the right place or not. 
Sorry this may be a bit long winded but I'm trying to answer all possible questions before they are asked.

I've moved into a ex family members house and I'm tracing wires that are no longer connected at one end. I came across an alarm wire that used to have a door magnet attached to it which triggered when the door was opened. However this wire no longer has the magnets connected due to the door no longer being there (it was to a shed that has been removed) the ends have been isolated and the cable has been coiled up out of the way. 
I have been able to trace the wire and remove it from the alarm box with the help of an electrician, however once the power was restored to the alarm it set up a fault which I found out to be an issue with the back up battery. I have replaced the battery, however I have lost my "unique" alarm password. I found the user manual (see link) which has the engineers code in but it refers to a "User Guide". https://www.manualslib.com/manual/2801115/Nexus-8.html?page=9#manual

When I google "nexus 8 alarm user guide" I get the following information User Guide but this doesn't seem to tell me how to reset my pass code.

Does anyone know where I can find the user guide that gives me this information or can someone give me step-by-step instructions as to how I can reset this please as currently I'm without an alarm as I don't know what the factory default code is so my electrician has turned the power off to the main alarm so it doesn't activate while I'm at work and it can't be shut off.

Thanks in advance to anyone who can help with resetting my pass code.