OK. I have had a lot of help from Andrew, but this app does not work well - at least on my PC. The usage calculation seems just to be a permanently increasing number that sets of the alarm when it reaches my quota (10gb) in around 10 minutes, with no actual internet traffic at all. Even when Internet traffic only is selected in 'Type of Traffic to display', it clearly is not capable of filtering local traffic as the I can watch the meter racking up usage when I transfer a 4mb file from my PC to my NAS server. Also the display seems to be permanently showing uploading and downloading activity - which I don't understand.
I have now uninstalled TBB and installed NetWorx. This does filter out local traffic and the displayed internet only usage looks realistic and does not increase, even when I transfer a 30mb file through my network. Unfortunately I still prefer the TBB interface and will be looking at it again in later revisions.