Entitlement of rebate on debt to RBS?

pimpypants

In 2009, I managed to repay my debt to RBS. This consisted of me going over my £100 overdraft as a student and I suffered £3,500 worth of charges for it. I made an arrangement with the bank to pay £100 a month to clear the debt. Despite asking for, I did not receive at any point, statements to show how I was doing with this arrangement. When I agreed to make the payment, the RBS employee, who was lovely, said if I could repay earlier, they could give me a rebate. In 2009, that's what I did. I phoned the bank's debt Gulag headquarters and told a disinterested young man that I would like to pay my debt. I also asked how much of a rebate I would get. He said, "We're not doing them just now". Recently, whilst looking at something on this site I suddenly thought; "Hang on, that's not right", it isn't is it? I have some paperwork which shows some of the various charges and a letter which confirms my arrangement to pay the month each month. What can I do about this, if anything? I realise I have been naive here but I would be grateful for any kind of advice.
Many thanks, Lis
I have started a new thread as I checked the specific one for RBS and there wasn't anything recent on there. Apologies if this is something I shouldn't have done.