Agh!
Another e-mail from Apple to say my payment has failed, and after a couple of attempts to try a different card called Apple, again. Paul this time, who said he could manually update the payment card (maybe it was Tim Cook I was speaking with last time!) which he did and the new payment card is showing. Still no change in status, but hopefully the next time they try the card it will be successful.
Weird thing was the 4-digit card number showing on the order wasn't actually any of mine! Will remember not to try and use Apple Pay next time
Did pop into the store in Glasgow earlier and checked out both blacks side-by-side. Still happy with my JB decision, which looks stunning!