If they were bought in one lot you need to open a "not as described" return rather than a "not received" case. So start a return on here:
https://www.ebay.co.uk/help/buying/returns-refunds/return-item-refund?id=4041
select "Doesn't match description" and if necessary escalate it to eBay after 3 complete working days.
The seller will still have the option to send the missing item or issue a partial refund but do not close your case until they do. You won't be able to open another one. If items are to be sent back seller pays for the postage label which you will receive via ebay.
