I had the exact same problem. Requested refund, waited 48 hours then it went back to same page. Tried it twice then eventually called them.
1hr and a half on hold but eventually got the lady to do it on the phone for me and she told me I earned £5 interest (silver lining) still waiting on it to be paid 10 days later but it got me past the stage you’re stuck at.
Give them a call and tell them the issue, they check your account for any reason it might be doing that (there won’t be) and they’ll do it over the phone with the bank details you give them.
Give it 2 days after the call and your account should change and no longer let you try to claim the amount yourself.
They’re very quick to send a letter when you owe them, but if they owe you they don’t half make it a palaver haha.
Good luck mate, unsure if you can message or reply to this as I’ve only signed up to reply to you but if you can message me if need any other info