The buyer is too late to claim a refund through Ebay and you are too late to claim from Royal Mail.
I would message the buyer to say that you are sorry they didn't get their item but there is nothing you can do.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)