There's no point repeating what other viewers have said - this is one of the great lost classics of BBC television.A stunning piece of work, and the climactic scene is both erotic and horrific.
It may not originally have been "A Ghost Story for Christmas", but I'm sure it was shown in that slot at some point in the 80s.
There was a companion piece made in 1987 - again with Charles Gray as narrator - this time based on an incident the life of Italian painter Cariani. It was called Cariani and the Courtesans, and starred a pre-Withnail Paul McGann.
It lacks the atmosphere or the horror of the earlier piece, but it's worth a look if the chance ever presents itself.