Have you checked the tracking that it's shown delivery ? For Royal Mail go to Track & Trace online to check. If it does show delivery somewhere on that case you will see the option to escalate, ask eBay to step and they'll force the refund.
If not, as the 28 days are not up, contact eBay Customer Services give them the tracking of the item sent back.
This is the easiest and quickest way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Lines open 8 a.m. - 10 p.m. on weekdays
9 a.m. - 6 p.m. on weekends.
Automated agents will be available on chat outside of the above hours.
I recommend contacting CS first thing in the morning as there's more chance of Dublin answering.
@11cromcro_23