If it was on a single order you need to open a "not as described" return, rather than a "not received" case. Click on "Return this item" on right of your order on your Purchases page and select "Doesn't match description" as the reason. If necessary escalate it to ebay after 3 working days. You will be issued with a paid-for return postage label.
The seller will also have the option to issue a partial refund without return or send the missing item but don't close the case before they do. And if they delay be sure to escalate your case within 30 days.
If you want to contact ebay you can use callback or the live chat link on here:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid