The cause is your back-up battery (which has a life of only 3 to 5 years) is dead and needs replacing. Before replacing you will need to reset the NVM.
Ideally, you need to download the manual to do this. It is available on the web via various sites. Search "Accenta Mini 8 manual + pdf".
Then follow procedure to reset NVM (Non-volatile memory). This is basically. Disconnect power (you can pull the fuse in the CPU control box (master unit). Then disconnect any links to connectors 17 (PA) and 27 (Set). Connect a link between 17 and 27 then reconnect power. This resets the NVM. Reconnect any original links (to 17 and/or 27). To reset the Tamper warning, press 'reset' button on the control unit (i.e. in your hallway). Enter 'User Code' if need be.
~ All done ~ Except you need to replace the lead acid battery in the box. Maplin do a replacement.
Paul D, August 2009