Has the buyer opened a case?
You must make sure you have added the tracking which confirms delivery in the UK to the case.
Then, contact Ebay, remind them it is a GSP sale and that you have proof of delivery within the UK, and ask them to handle the refund.
The link below will get you a callback.
https://www.ebay.co.uk/help/eua?id=5190
"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)