You should check the Windows update history section to see if any Microsoft updates coincide with the dates in question.
I think the last large download, ie the latest Creators Edition was under 4GB download.
I am not sure if when an update download is started and you turn off your computer during it whether it stores that partial download on the computer and resumes where it left off the next time you start the computer or if it starts the download from the beginning again.
If it is the later it could mean that you could be repeatedly downloading large amounts of data until the actual download is completed and installed.
thanks for your feedback
yes I do plan on visiting my father asap and checking the windows update history of his pc
watch this space
I did a bit of checking and apparently a Microsoft download will pick up where you left off should you turn your computer off mid-download so this should not cause multiple attempts to download and any subsequent excessive data usage.
Unless I have missed it you haven't said what type of broadband your father has. If he is on ADSL (normal) and not Infinity you should do a check of how long it would take to download the amount of data that BT have said he has used.
It may be that depending on the download speed of his line it would not be possible to download that amount of data in the time scale that BT have said he did.
I would get back onto the moderator and ask for a detailed breakdown of the data used including dates and times.
Given that he has said it was due to Microsoft downloads, it would appear that he is able to access that information. This would allow you to compare, as much as possible when your fathers computer was switched on and more specifically what was downloaded or at least how much data was downloaded and from which website.
If this information is not available or forthcoming you should lodge a complaint regarding it and if not satisfactorily answered ask for a deadlock letter. This should specify BT's position on the matter and will allow you to take it to the Ombudsman.