Debited incorect amount

On 24 July 2015 I downgraded my services but when the debit order went off they still charged me for the previous package. I would like a refund of the defference between the two packages. I phoned the call center and was advised to go to the bank to reverse payment but 1) it is going to cost ME bank charges 2) it is not guaranteed to be successful 3) it is going to take too long. When I phoned the call center about a month or two ago and asked if the 24th is too late to downgrade and only be charged for the new package I was told no. Please assist in this matter as in today's economy most of us live according to a strict budget. Hope you can understand.