If the buyer hasn't got their item you were going to have to refund anyway so best do it before they open a dispute.
After 15 working days use your Proof of posting and Form P58 to claim from Royal Mail.
Send the buyer a friendly message, apologise for their missing item, tell them you have refunded and ask if it does turn up that they contact you to either repay or return if they no longer need it.
You are now reliant on the buyer's honesty so best keep communication pleasant and friendly.