No point ' presuming' if it has been delivered, you need to know if tracking has shown delivery.
Check Royal Mail / courier's website, and if it has , enter the tracking details if not already entered, into the case details and it will close, no need to refund.
Was this a Simple Delivery transaction? If an item is lost and case opened , eBay should deal with it , and if tracking hasn't shown delivery, they pay for the refund and you get to keep the payout.
This kind of case can only be closed either by the buyer, or with a tracking number showing delivery.
@rachalex31