slowly getting to the bottom of this...
The Live Traffic panel seems to be one of things that is tripping the mod_security - it doesn't like the fact that "Live Traffic" is calling admin-ajax so many times.
It looks like the host's mod-security is being excessive pernickety about admin-ajax being called every 2 seconds! Reducing the amount of calls that the scripts call or use admin-ajax has hopefully resolved the problems!
FYI (and for anyone else that might have the same problem) - there is a setting in Wordfence's Options panel (Update interval in seconds (2 is default)) which allowed me to change the interval time.
Thanks for your help.