It's actually hidden in the new UI, there was no button that would let me do it, I managed to go back to settings and re-enable the classic UI, then in the view transaction details page I found the missing refund button.
The beauty there is you don't have to win, to win. A £5 dispute costs them more than you, when you get to control the venue.
Once upon a time I shared this sentiment about PayPal, but history has shown me that people continue using them anyway. I mean, every person I talk to has personally experienced a PayPal horror story or knows someone who has experienced one first-hand, and they continue using PayPal anyway. Some feel they don't have a choice, others just don't care, but I no longer feel like making noise about the PayPal horror stories is going to have any impact on them, so I encourage you to contact the ombudsman/FCA regardless.
Edited to correct to FCA - https://www.fca.org.uk/