Hi Mc,
If it helps, your not on your own. I see like you and i there are others in the same situation.
A colleague of mine was recently hit with an £900.00 charge for 3gb worth of extra data she didn't use. Yet when you try to contact BT an explain to a different person for the 10th time and they give you yet another case number because the person before them didn't entire it correctly, yet that department dont deal with it; to be told due to data protection they aren't allowed to tell me what sites I've been on sites!! Yet another rep tells me they can but the charge must be a software update but still will not disclose what.
Probably like yourself we have emailed, rang and now sent a letter off to them signed for. Unfortunately because we are not a small company we aren't able to go through the Citizens advise or the ombudsmen, but you may be able to?