If they were bought in one transaction start a return on here:
https://www.ebay.co.uk/help/buying/returns-refunds/return-item-refund?id=4041
select "Doesn't match description" or missing item and if necessary escalate it to eBay after 3 complete working days.
Seller will have the option of sending the missing item or issuing a partial refund but do not close your case until they do.
