The system installed is a Cooper Security/Scantronic 9452 - Obsolete. Alarm systems usually have 3 power sources: -
Main control panel (this is where all the devices wire to) = Mains & internal battery.
Extarnal bell = supply from main alarm + Internal battery.
If you switch the mains off and disconnect the battery in the main panel and external bell then that should kill your system.
If you want a profesional alarm company to look at your system either contact the original company or goto www.nsi.org.uk to locate local companies.
Du Ville
June 2009