You are right to change the backup battery, they should be changed about every 3 years,Screwfix direct sell the batteries for less than a tenner.
I am not familiar with the veritas procedure for batt replacement but usually it's connect mains first, then battery,followed by enter your security code to silence alarm. You can get a user manual for free from :-
http://alarmhelp.net/downloads/v8c8r8userguild.pdf
you will need acrobat reader to view it.
The bleeps you are getting just might be a maintenence alarm , if that is so you will need the engineer code to clear it.
I hope this helps you !
Pete
July 2007