Little Mix
From Tardis Wiki, the free Doctor Who reference
Bill Potts had a whole playlist on Spotify with Little Mix. One of the songs on the playlist was "Black Magic". Shireen told Bill they refer to talk about her taste in music. (TV: Knock Knock)
Behind the scenes
Music by Little Mix also plays throughout the opening scenes of the episode, where Bill and her mates are searching for a house to live in.