So, I’ve heard back from customer support and as promised I’m updating the thread:
As this is a quasi-cash transaction it seems like your issuer is declining these payments, as you mentioned this is not an ATM specific issue and we are approving these transactions.
As you have already contacted your issuer via telephone, In this situation, we would usually advise getting in contact with a representative in a local branch however as this is not possible at the moment the best alternative is to use a different payment card linked to your Curve account or use your card directly as we have no control over what your issuer accepts.
Not sure how I feel about this, tbh. I was trying to get away from carrying multiple cards. Now it seems like I need my cash card and a credit card (as a failsafe) so I’ve ended up with a third card to take around with me!
I don’t know what else to do. My bank (RBS) doesn’t have many branches where I live (not, erm, in Scotland!) so I don’t have a great deal of optimism to commit the time to travel to a branch and resolve this (plus, I was talking to a specialist from the retail fraud department, not a customer adviser in a branch). I may try calling again I suppose.