Not collecting return items

I have had a terrible experience when ordering online from Cotton on. My order was delivered late by 3 days only to find out that they gave me wrong sizes when I asked for a return my query was thrown from one service agent to the next all saying “ I will escalate the matter with the courier company”. I asked for a manager to email me so that I can raise my concerns, however that did not happen. Till to day I am still sitting with an online order which was to be collected beginning on Dec