Sorry, we couldn't update your payment details because...
1. Our payment gateway is currently unavailable so we are unable to authorise your card. Please try again later.
Why? What can I tell my clients?
As the message suggests, this was a problem with the payment gateway, rather than Campaign Monitor itself, which does make it a bit trickier. Once we realised we were able to put in place a workaround system until the payment gateway people could get their side working again.
Sorry for the trouble, all is now going through fine.