Return errors & Delays and poor customer service

I made a order with cotton on, got delivered and the size was way to big. I made an enquiry with the customer service team for a return, that way I could just get a smaller size. They told me that Dawn wing will be in contact with me for collection. 2 weeks has passed and after many emails cotton on responded to me that Dawn wing said they have already picked up my parcel, which is not correct because it is still with me. The customer service team told me that I must phone dawn wing to sort it out. How is this my responsibility to sort out? I phoned them this morning to arrange for collection. This is just unacceptable as I needed the pants I ordered for a certain time that I was going somewhere. I would not recommend ordering online. Very very poor service and communication.