James - thanks for the welcome.
Perhaps it wasn't clear, and as the poster above - I already have an online account (I am also a mobile phone customer). The account page has a nice drop-down box containing the two mobile numbers (my phone and my mobile broadband numbers) but unfortunately there is nothing "at my fingertips" pertaining to the mobile broadband.
Whenever I try and select the "detailed bill" or "unbilled charges" (or indeed whatever I try) it reverts to my mobile phone bill.
"Approximate" figures are really not enough. Much as I like the MB service I have no intention of paying you £15/GB for it if I go over my limit
So I need to have decent track of what's going on.