Mysterious 'Data upgrade charge'

[#1]

Last month was pretty heavy for me in terms of usage.  I expected to pay more this month, as the bill would have worked out at about £37.00.  Imagine my shock then when I logged on today and found this:

Qty Description Unit VAT Gross
1 Account Billing (2010-09-06 to 2010-10-05)                         £17.01 £2.98 £19.99
1 45GB Included Data Transfer Charge From 06/09/2010 £15.96 £2.79 £18.75
1 *Include Data Transfer Upgrade Charge On 06/09/2010 £15.96 £2.79 £18.75*
Net Amount £48.93
VAT Amount @ 17.5% £8.56
Invoice Total £57.49
What the hell is this, 'Data Transfer Upgrade Charge'?, I have not been informed about this at any point, via email or otherwise. I haven't changed product either.  Please refund this £18.75 charge as it's clearly a billing error, I've never been 'taxed' extra for more usage before.

[#2]

I have removed a number of posts.
Let's wait for a member of staff to pick this up

[#3]

I'll have a quick look at this now...
Edit: You increased your included package usage to 45GB during the last billing month. This costs £18.75 extra. As we bill you in advance, we waited until the next billing date to bill you for the usage in the previous month and the next month. This is how the system is intended to work. As you used 43.54GB last month, I can't refund any from that period. What we can do, however, is return your account to Pay As You Go usage which costs slightly more but bills at the end of the month for any extra usage that was accrued. If we do this, I will happily refund the extra £18.75 but if your usage stays at the same high level, it will end up costing you more on the long run.