@gavs82008
Broadband - I don't have TV services. That said, I did take a look at the TV payment detail update page as well, a little later and I think there is something odd with the web forms. The Broadband/TV forms look different; the TV one, however, behaved as I would expect...
...and, on a hunch, it turns out heading to the Broadband page immediately afterwards must have carried some client-side values over for some reason. The billing address fields were pre-checked (same values as attempted previously), and the confirmation button was already enabled. As a result, I was able to submit the form.
My card was verified with a small £1 charge, authorised through the bank's app - however, then the web form gave a billing address error when trying to redirect to the confirmation page. I gave up and went to bed.
Happy ending, though: this was a couple of days ago. I've now rechecked, and it looks like the details have been updated as a result of all of the above (or perhaps copied over from my TV payment details section). Or perhaps my bank has chosen to update them again in line with my old card's actual expiry date. Fingers crossed that this month's payment will be successful.
It's going to be a difficult workaround to navigate from scratch for the next person to stumble upon this post. My browser is Firefox, if it helps narrow things down later.
