Apologies for not posting an update.

I did call the oyster helpdesk, who have refunded 7.15 (I was charged 4.45 + 4.45 = 8.9, I should have been charged 1.75, so they refunded the difference)

I then asked them whether they could provide an explanation, replying that it’s because I have exceeded the maximum journey time limit (although I don’t think I had)