You definitely need a new battery - type 12v 1.9amps (you may be offered 1.8amps or 2 amps, both are ok and the manufacturer doesn't matter)
Take the measurements of the existing battery and go to Maplins or your local burglar alarm shop (yellow pages) and they will sell you one for about £20.
Replace the battery and things should settle down.
Gerry
November 2009