Try talking to customer services and see if they can help you.
Here's a link to get a callback that should bypass the automated assistant during opening hours.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
If they can't/won't help and you paid by PayPal open a case in their resolution centre.
They will probably want proof the item was returned so hopefully you sent them back by a tracked method.
Otherwise contact your bank or debit/credit card provider and dispute the payment by initiating a chargeback.