To make The Beatles: Get Back, director Peter Jackson was given access to a staggering 60 hours of footage of the band recording their 1970 album Let It Be. The documentary’s emphasis, of course, is the Fab Four – John, Paul, George and Ringo – but, on the periphery, Yoko Ono, then aged 36, is a constant, subtle presence. Wearing all black and then all white, there she is; reading a Beatles fan magazine, mending some clothes or eating a pastry.