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I did manage to get an answer on twitter from the bank. At first they had no clue what I was talking about, I think they mistook the situation and thought that the whole credit card account had been closed, which is not the case. After much to-ing and fro-ing I think they got it, and said that yes, it should just go into the account.
Trying to get anywhere with air bnb was very challenging. Again because I don't think their live chat people have english as a first language. They must have some kind of translator but if its google translate, frankly i don't wonder they can't work out what you mean.
Air bnb's policy for anyone else who might come on here and wonder about it, is only to return to the card the booking was made on. Even if that card is now invalid for whatever reason, it can't be changed. They say to prevent fraud, which I can see, but given that you have to log into your account using a password, why you can't make this change yourself, using all the codes and information that you set it up with I don't know.
In the end, their live chat person put through a refund, even though I didn't ask them to, to the invalid card, so I am now waiting events. They also, in error, put through a payment, also to the invalid card, which obviously was declined. They keep trying to take this payment, even though the booking is now cancelled!! There is something not quite right in their system, which has surprised me because previously I have never had any kind of problem with them.
And all this because of a silly little virus with the potential to kill!!