SCANTRONIC ALARM BATTERY?

JOHN THE ALARM ENGINEER IF YOU ARE READING THIS.
I've been told the fault on my Scantronic [on this listing of questions], that my alarm battery needs replacing. I checked it and it only reads 6.8v and dropped to 3v after mains power down for a day [couldn't stand the siren going off ] Have tried a few alarm companies who sell components and none can help with the LT 2.2-12volt battery. I have seen one from Maplin of Brighton, [I'm in Worthing] on web 2.1 -12v which looks very much like my existing one would the 2.1 be ok to use? as 2.2's seem very scarce.
Many thanks in anticipation. Bob