Hopefully you haven't refunded yet.
You need make an Actionfraud report asap and let eBay have the Actionfraud reference number and let them know that you've received a different item back to the one you sent out. There should be a "Report a problem" link in your return case, so select "Different item returned" as the reason and add the Actionfraud reference number and information.
I'd also contact eBay and go over this with them. I’ve found that the easiest route to contact eBay Customer Services is via the link below, you can select to talk with them via live chat or request a call back:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid