How to put a complaint into xbox about non refund

Hi I've been having a nightmare with returning a design lab controller. They received it on 29th October and they lost it so never refunded. I have rang on numerous occasions got some of the money back not correct amount and been ringing over and over
trying to get rest back. Keep telling me not to buy a design lab again and to buy elite. But still not received money don't know who to go to next to see about this

:)