Replacing battery pack Veritas R8?

My alarm has started intermediately bleeping/humming during the night more noticable from the outside, I've checked the power pack unit on the wall downstairs and its displaying red.
I'm under the impression that the battery may require replacing. I've taken the screws out of the front panel and an alarm sounds (tamper proof alarm) which sets the tamper light on the control panel.
My question(s) are:
1) Do I turn off the mains power for the alarm before changing the pack.
2) Do you know where I can get a replacement battery from i.e. B&Q
3) When I've changed the battery, will the alarm automatically reset i.e. tamper light go off on the control panel or will I have to enter an Engineer code.

The alarm i think was fitted back in 1996 and the battery replaced in December 2001. The previous owner hasn't left any documentation regards operating or maintaining the alarm. So any help will be appreciated.