Confused about final bill

[#1]

I migrated away from Plusnet on 22nd September and was under the impression that (as my month starts on the 21st) that I would be charged for 1 or two days via Credit Card rather than a direct debit.
I have now had to pay a full month (£16.99) covering the period (according to the bill) from 21st September to 21st October.  The person I spoke to said that broadband was charged in arrears and that the bill was wrong.
Could anyone explain what should happen now as the note on my account already "warns" me about October's payment.

[#2]

Hi,
Sorry for the inconvenience, you should not have been charged the full bill, as you only had service for one day.
I have refunded you £16.42 for the difference in where you shouldn't have had service. I will also cancel your account to ensure no further bills are issued.
Matty