Ask the buyer to open an 'item not received' case. You can refund them straightaway and you will get your seller's fees back. You have to refund the total amount they paid, including postage.
You will have to claim from Royal Mail, through Ebay/packlink, after you have refunded your buyer.
In theory, you should be able to get back the postage cost and the value of the item sold. I assume you used RM Signed For ('recorded delivery' has not been a thing for years) which covers then value of an item up to £20.
Details for your claim should be found here:
Packlink Help centre
"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)