How can i tell how much data i am using a month? Is there a section on the bt website that i can log onto and check?

Your hub home page will have the upload and download usage.
It is one of the hubs features.

All hubs have the same address.

Three ways (i know) to get to your home page ..

http://bthomehub.home/

Or type this in to your internet address bar at the "Very top of the page " you know the one that
say http//uk.answers ( not the search box)

192.168.1.254

Or make sure your network discovery is on then look in your network.
and click on the icons ,one of which will be your router.

If you would like any more info on the BT hub i have found this site of great use.

http://www.frequencycast.co.uk/homehub.html