Thanks, all, for earlier comments, which revealed some interesting variations in recruiting culture across organisations. Candidate A’s second referee just replied with a sufficiently positive write-up that I feel confident in making the offer. Checking the records I find that we had already asked Candidate B to help me reach their referees.

As it happens, Candidate A was marginally the front-runner after interviews, so I will tell Candidate B, perfectly truthfully, that unfortunately there was a better candidate on the day, and incidentally they need to re-check the coordinates and availability of their referees.