Never had one before, but got a mail from bt saying i went over my download usage limit, if i go over again i will be charged

Now how can you the customer be sure of their download usage or least monitor it & how can i be billed without record or proof that i have gone over?? like you would get a breakdown of costs or usage on other bills or mobile credit but not specifically internet bills, specially when there could be wifi capability & be sure that those in household are only ones using your BB & not a piggybacker hammering your usage.
How do they work it out or how do they monitor what you are using & why isn't that available for customer to monitor instead of a mail from bt all of a sudden with no idea till they go over

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