I had to buy a ticket with Swiss Air and it asked me to either pay with Credit card (Visa, Mastercard, etc) or with Debit card (Maestro, etc). I selected credit card because the Monzo card says Mastercard on it but it told me it was the wrong type of card. So I tried debit card with Maestro and it said it was not a Maestro card.
So I could not actually pay with my Monzo card
In the end I used a back-up credit card I have.