I went to the premiere at Tate Britain of the film Frank, a portrait of Britain’s greatest living artist (and famously publicity-shy) painter Frank Auerbach by his documentary-maker son Jake. It offered a unique insight into the artist’s working life and was followed by a question-and-answer session with Jake, who responded to several of the enquiries by saying he could not answer for his dad and insisting he knew no more than those who had just seen the film. Rather brilliantly, however, Auerbach Snr – who could have answered all the questions in mesmerising detail – was sitting inconspicuously in the auditorium listening to and watching everything that was going on. It was a rather wonderful example of the truth behind Edgar Allen Poe’s story, The Purloined Letter. Nothing is so invisible as when it’s in plain view…