I just got off the phone with BT.
I have been having issues with my monthly usage - they say I have neary used all my 40GB allocation last month.
We got to the bottom of it. Apparently if I stream video from my PC to my media server by my TV (across the LAN, not downloading from the internet) this comes out of my monthly usage allowance.
I couldn't believe it.
Oh well, only another year to go on the contract.