Refund him the money and tell him to return item on delivery (or pay money). If it doesn't arrive, claim £36 from royal mail.
Using unrecorded delivery on ebay is stupid. Since you can get scammed easily. Doesn't mean the buyer is lying though, you can never know. Considering the recent strike and cases where they are sorting normal post out on streets, it could easily go missing.
Also, it's upto you when to start using special delivery. I personally on items worth £90+ allow only special delivery.
Anything less, I think about whether its worth paying the premium over second class recorded. I've never had anything go missing in my life out of hundreds of things sent.