A fellow Costa victim, sorry, customer has gone back to Vacations To Go, through whom he booked his cruise and with whom, therefore, he had a contract.
Disappointingly, they have declined to accept any responsibilty for the 'ruined cruise' (© Salvatore Donato), saying, 'They (i.e. Costa) are not going to work with us about shipboard issues so I encourage you to continue to work directly with them.' In other words, Costa have told them to get knotted and they've said, 'OK, fair enough'.
I've complained direct to Virgin through whom my wife and I booked. Let's see if they take a more ethical and more assertive stance.