As you are with BT you can call them any time and ask for a detailed account usage .............assuming you get put thru to the right person!!
I used to be capped, but talked it thru with someone at BT as my kids started to be very keen on tv and movies over the net.
It was obvious from his usage data that I needed to pay for the unlimited service.
If you are with BT Broadband there is a usage monitor.
The charges for 1571 and caller display are a disgrace. I've got rid of caller display, but can't be arsed to get the answer phone working, so I stayed with 1571.