I found Music Upstairs a slightly weird book and quite hard to get into. It is set in the 1960s and is about two girls, who live in Earl's Court, renting a room from the couple who own the house, Pam and Lenny. The novel is mainly about one of the girls, Sidonie, who becomes romantically involved with both Pam AND Lenny, whilst pretending to have a secretarial job. In some ways it is classic bedsit literature, but I didn't really "get" it I don't think.
It was Mackay's second novel, after the other two novellas which comprise the other Mackay VMC on my list; Dust falls on Eugene Schlumburger and Toddle on the Run (she has written a number of other novels which I believe have been published by Virago but not as VMCs). She wrote it aged 19 after living in Earl's Court for 2 years and a long period of playing truant at school.
Two covers here for your delight...firstly the original green one, which is the one that I own, and then the more recent one.