Well I hope I've just fixed our faulty alarm system.
http://www.cableforum.co.uk/board/35646450-post1.html
Today I thought I'd give it a go so decided to inspect the internal battery. However, as soon as I touched the control panel to unscrew the 4 top cover retaining screws, the tamper went off which shouldn't have happened and was as loud as it was unnerving. After removing the cover, I soon found the tamper switch and could see that one of the 2 small plastic prongs which holds it in place was missing thereby allowing it to rotate. After a lot of looking and some trial and error, I also discovered the black wire from the switch to the circuit board was loose and fell out as soon as I touched it. After reconnecting the wire, I managed to secure the tamper switch into the closed position (as an interim measure) with a cable tie thereby curing the permanent tamper fault and alarm which was doing my head in. I then set about replacing the battery. Sure enough the positive terminal was corroded and stuck in place but after prising it off I was able to clean up the connection and fit the new battery without any problems. I screwed the control panel cover back on and successfully armed/disarmed the system a few times. I'll allow 24 hours for the new battery to charge and then try turning off the mains power supply to see if backup power has been restored. Fingers crossed!....