The presence of Mary Tamm marks this as, most likely, Christmas 1978. The slightly more iconic and long-lasting regeneration of Romana was, of course, played by Lalla Ward after Tamm left the role in 1979.
Ward married Tom Baker towards the end of her run, but it didn’t last (ah, “showmance“). She’s been married to Richard Dawkins since 1992.
Tamm, sadly, is one of the few companions as yet promoted to the Choir Invisible. She died of cancer in 2012 at the age of 62.