BT = a Bit Thick

My telephone bill is drawn up on the 8th of the month for all calls and broadband use over the preceding month.
On Aug 8 my bill was £6.05  for calls made.  I had not exceeded my 10gb/month allowance so no additional broadband charges were due.
On Aug 22 BT took this sum via the direct debit mandate.
On Aug 23 I received an email from BT saying that my  broadband usage in JULY was - -----wait for it ------ 318gb
Now, on the BT website there is a useful function which I view from time to time called My Broadband Monitor. This is updated at midnight each day and at no time has it shown broadband usage anything like this figure. As it happens I have printed evidence of this.
When I first contacted BT customer service to query this email, I spent two hours being passed from one department to another but achieving nothing.
Taking a different tack  I spoke to the accounts dept and they confirmed that my bill of £6.05 was correct and included all my phone and broadband use up to Aug 8.  When I mentioned the email I had received the gentleman burst out laughing and advised me that it was not genuine and I should ignore it.  Being a suspicious character I did not take his advice and wrote to BT asking for clarification.
Their response was that I really had used 318gb in July.
The saga goes on   and  on   and on   and on BUT
BT cannot explain whey their system was unable to show this amazing broadband use until three weeks after its occurrence.
They just say  "you used it and must pay for it"