Well, that was odd. Didn’t need to call in the end.
At some point during the course of today, unprompted, TfL amended the refund of £4.10 (which had been applied to my charges for 4 August, which they’ve still yet to take as far as I can tell), increasing the refund to £7.25.
They’ve shown their working for this: when I click on the £7.25 “amendment to past fares” credit it takes me to a page showing that I was originally charged £22.15 (which is 5p more than I was actually charged, but I’m going to let that go as I’m sure the admin of trying to give them their 5p back would cost them more than 5p to deal with). It then shows the amended charge as £14.90, the applicable daily cap, and calculates the refund as £22.15 less £14.90 = £7.25.
If I have to request a refund again I’ll give them a few days to sort themselves out before worrying about it too much. All looks fine from TfL’s end now. Will check my credit card account in due course but should be fine.