Long story - here's the summary:
It turns out that the big credit card companies (in my case, MasterCard issued through JL) send a file containing the new credit card details to their big customers ("for example, Mr Goldstone1976, Amazon & Uber") at the same time as they send you your new card ("typically 4-6 weeks before the expiry of the existing card"). It's then up to the big customer to upload your details to their system at a time of their choosing, which could well be before the expiry date of your existing card.
In my case, it turns out that Uber have been trying to collect a fare from my credit card since 15/6. JL have been declining it because I hadn't activated the new card, and Uber had already updated my card details in their system. Card details that I hadn't given them.
Morale of the story: activate your new card and swap all your payment details pretty much as soon as you receive the new card.
Bigger issue: wtf are credit companies doing, giving my card details to anyone, at any time?
It turns out that the big credit card companies (in my case, MasterCard issued through JL) send a file containing the new credit card details to their big customers ("for example, Mr Goldstone1976, Amazon & Uber") at the same time as they send you your new card ("typically 4-6 weeks before the expiry of the existing card"). It's then up to the big customer to upload your details to their system at a time of their choosing, which could well be before the expiry date of your existing card.
In my case, it turns out that Uber have been trying to collect a fare from my credit card since 15/6. JL have been declining it because I hadn't activated the new card, and Uber had already updated my card details in their system. Card details that I hadn't given them.
Morale of the story: activate your new card and swap all your payment details pretty much as soon as you receive the new card.
Bigger issue: wtf are credit companies doing, giving my card details to anyone, at any time?