TBBmeter is a great tool for measuring usage, but may not cope with Xboxes etc. Fellow non-techs might like to note how to access BT's own monthly record from which you will be charged:
Go to My Bills and log-in. Select My Services. Click on the broadband icon. Below 'Manage broadband service' click on 'view BB usage'. Up comes a slider with green bar representing your usage, and the current figure.