Without sounding too much like Donald Rumsfeld, with his “known unknowns” and “unknown unknowns”, the future calendar of a great actor consists of unwritten parts and written-in roles. Somewhere, dramatists are writing scripts yet unknown which – as Charlotte Jones’ Humble Boy and John Hodge’s Collaborators did – will become peaks on the CV of Simon Russell Beale. But, equally, we have known ever since we saw SRB play Hamlet, Richard III and Iago so brilliantly in the early part of his career that he was a Lear in waiting.The wait ended last week on the Olivier stage of the National Theatre. Some critics have suggested that the 53-year-old actor is too young to portray a senile king; but Russell Beale is already more than a decade older than