The queen of that court is Michaela 'Micky' Cazaret, played by Ava Gardner, whom we learn is so rich she has no need to do anything but indulge herself in pleasure and enjoyment all day every day, and has gathered around her a court of young people to accompany her on the ride. They have no obligations to her for as long as the arrangement lasts, but when she tires of them and tells them to leave, they had better do so - or her personal assistant, Elroy, will soon ensure that they don't trouble her any longer. The role draws very effectively on Ava Gardner's real-life image as a high-profile film star of the previous generation, plausibly very wealthy and still beautiful, both of which give her power and authority over her court, but becoming increasingly insecure about her age in comparison to the twenty-something Beautiful People with whom she has surrounded herself.
The Tam Lin figure is not exactly hard to spot - he's called Tom Lynn (played by Ian McShane) and begins the film as Micky's favourite lover, but incurs her wrath when he meets and falls in love with the beautiful and virginal Janet Ainsley (Stephanie Beacham). Stephanie, incidentally, isn't the only connection this film has with Hammer's oeuvre: Joanna Lumley, Jenny Hanley and Madeline Smith are also all present as members of Micky's court. Things play out much as you might expect if you know the ballad, including motifs such as Tom getting Janet pregnant, her seeking an abortion, her picking up a double-stemmed rose, and a wild hunt at the end in which Tom drives a white steed (car) and turns into a bear, a serpent and a flaming brand while Janet has to hold him tight until he returns to humanity.
According to Wikipedia, it is the only film Roddy McDowall ever directed, which is perhaps a shame, as he seems to have done a very good job - though in fairness it would only mean he had done less acting if he had done more directing, and I don't think anybody would want that. The whole thing is available on Youtube here, which is how I watched it, and I can attest that it is a good enough print to stand being cast to a large flat-screen TV.
Highly recommended to anyone who likes folk horror and / or films about Beautiful People getting high c. 1970. The two have always been more or less synonymous anyway.