I think I’ve overpaid and claim is coming to an end

I have been paying via DEO £469 every four weeks. This consists of £352 payments + £117 arrears. My child turned 18 in October 2015 so I enquired to when my payment would terminate. I received a letter saying the final payment was September 2015.

I would like help with two points if possible.

Firstly what should be my course of action to bring the matter to a close?

Secondly, when my claim was set up the payments were made up to the child’s 16th Birthday. Then it was changed so payments were made up to the child’s 20th in some instances. As my arrears were originally due to be paid off by the 16th birthday, but I went on to pay 2 more years, should I be entitled to the claimextra arrears payments back, and how would I go about this?