On Royal Mail's Track and Trace online, does the tracking explain where and why the item is being held? If in the buyer's local sorting office then the buyer needs to go there to collect it, or to inform RM to redeliver at their requested date.
Even if attempted delivery has been made, so it's essential to have correct information from that tracking then you will not have to refund.
If you are seeing something like..... ' we will update you when delivery is made' cannot remember the exact wording then you will need to refund the buyer.
If the item arrives later, nothing can be done through eBay as the case closed with the refund, and you rely on the buyer's honesty to repay you. Most people are honest and would not want an item for nothing.
It's happend a few times to me in the past and the buyer was always happy to repay me.