This question has vexed the forums but the answer is very simple:
1. Make sure you have joined BT's on-line billing with your main phone/broadband account
2. Look up the direct debit reference number at your bank for your new BT Mobile account (starts GB********)
3. Go to you BT account and add account
4. Use the reference number and your Mobile number and there you have it -usage monitoring
Note: the monitoring lags by about 24 hours so beware!