Hi David
As Eric says you files arent going onto the net then back into your network, it should be just local traffic.
The network meter you have on your pc sounds like its measuring all traffic, both internally to/from your file server and the internet. if you log onto the homehub, click on advanced, enter user and password, then click Status, ADSL Line and it should tell you how much INTERNET traffic has come and gone.
Trust BT to try and sell you another package without even investigating the issue.