I think you have to ask whether this is worth pursuing.
In essence, this is a straightforward contractual dispute - BT allege that the charges are due under the contract in existence at the time, whilst you claim they are not. The time limit for legal action based on contract is 6 years (section 5 Limitation Act 1980
) so the matter is far from being statute barred, but I would argue that, if you eventually reached the point where court was the only option, taking court action based on these facts is disproportionate considering the court fees would be £25
(assuming any claim is filed online).
Any dispute will revolve around proof of the usage in the last month. BT will have the RADIUS accounting information showing the usage, which you have found no reason to dispute in any other month. You appear to have no evidence to support your dispute other than the recorded usage being out of line with previous months. In support of their figures, BT could well argue that you'd used an extra 15-20GB that month (hence the additional £20), which is hardly that heavy on an ADSL line, and could well produce statistics on complaints received each month in relation to usage to show there's no evidence of a widespread malfunction in the accounting data for that month.
Ask yourself - on the balance of probabilities (the civil standard of proof), would a judge be more likely to believe BT's contention that the data is accurate, or yours that it is not?
You have no right to demand particular review measures be taken unless your contract granted you those rights. BT appear to have checked the data at the time, satisfied themselves that nothing is wrong, but failed to contact you to let you know about this. That is bad customer service, but does not undermine BT's position from a contractual point of view.
Ultimately, I can't see any point in taking this any further. There are really no lessons that can be learned with the time that has elapsed, and you seem to have no evidence that rebuts BT's contention that the usage figures are accurate. Unless you stated that you were paying whilst continuing to dispute the sum owed, your payment indicates you agree the bill is accurate.