Hi mduncan, here's the response from the Mobile team:
‘BT Mobile data tariffs are designed to include an allowance of data usage that the user is able to utilise before being charged for incremental usage. This allowance is measured in volumes of data traffic passed through the network and is not time related. Therefore the latest versions of access manager provide users with the ability to see the amount of volume of data passed through the network during the current billing period.’
Does this help?