Current update:
So apparently there is a separate set of terms and conditions for Adagio which says that they charge the first night of a reservation as a guarantee. Okay, fair enough. Unfortunately, the Accor booking system doesn't seem to recognise this and gives the typical Accor flexible terms on the booking page. 'no prepayment required. Pay when you arrive. Etc.' So I put the room on my typical travel card that I have frozen when I am not travelling to protect from fraud. I unfroze to make the reservation and then froze it again, as I usually do. If I had known they required a prepayment, I would have handled it differently. The card was charged a couple of weeks after booking but it was frozen so it was rejected. The hotel then cancelled the reservation without informing me in advance. After I contacted Accor, they later got back to me to say that Adagio is a separate entity so Accor can not do anything about it and that my complaint would be sent to the Adagio team. The hotel has offered to make a new booking, but only for the current rate and on the condition that I pay 2 out of the 3 nights up front. I don't understand where this is coming from as it was originally meant to be just one. The next best option is about 25% more expensive, and not as nice, so I am not very keen on that. But a bit conflicted. The trip is towards the end of May, so will try to sort it out before then, but we shall see.
Honestly I am almost as annoyed with Accor as I am with the hotel for just throwing their hands up and saying, 'not our problem.'