Royal Court theatre, LondonMarianne Elliott directs Nick Payne's tonally uneven play about a missing son that comes with ill-fitting moments of comedy Nick Payne is an exemplar of this theatre's mission to nurture new writers. It was here he started out, on the young writers' programme. Now a starry alumnus, he completes a career full circle by premiering a play on its main stage for the first time. Is it a triumphan…