OK, I think I may have an answer based on an answer to a similar missing field question which I posted here.
It looks like this node is not available in the get list and you have to retrieve each appointment by its ID to see this information.
IMHO, Not very helpful since often in lists we want to see customer details. I guess this is a casualty of the "enhancement" of being able to have multiple customers per booking.
Unlike my issue which was documented, this behaviour appears not to be documented.