Well, found the source of my problems and it's all at my end. No problem with BT, no hackers or anything else exciting. Basically my flatmate was using bittorrent and - god bless her - didn't realise that after a download finishes the file continues to upload so long as you leave it listed in the client (like a good bittorrenter should...).
Had a little chat and convinced her that if she REALLY must download things using bittorrent then she should shut the client down when it's finished. Been keeping a beady eye on the BT usage monitor and sure enough, usage for the last week has been pretty minimal. We'll probably still exceed 40GB this month but we'll be well within the initial £5/5GB excess. Annoying we have to pay any excess but I think this'll be the end of it.
That said, I'm still installing TBB bandwidth monitor on all our PCs before next month and keeping an eye on usage from individual PCs. If hers use all the bandwidth again she can pay the excess alone.
So thanks for all posts and sorry to have wasted your time! OP signing off.