Thanks for the update.  I was lucky as I contacted my buyer & she was happy for me to pop in another book instead of a refund, so problem solved - this time.

Wish they would hurry up, as I quite often have buyers paying for multiple postages as the "request invoice" icon doesn't always work for combined postage.  A lot of problems with selling on Ebay!!

Have you reset your modem or tried using a different computer not using your modem. Sometimes I will use my mobile and switch off wifi. 

I have had no issues but have in the past but usually fix themselves after a day or so.

If you have a second account or someone near by does see if they can access refunds on your computer.

I was able to finally refund my buyer today.  Worked first time so hopefully it is now fixed.