The problem for Theresa May when she stands up in the Commons this afternoon to announce an inquiry into allegations of a cover-up of a child sex abuse ring, possibly involving senior politicians, is one of credibility. Not hers, which is remarkably unscathed after four years in the most exposed job in Whitehall, but that of the whole edifice she represents, the tight little self-preserving body of civil servants and politicians and Oxford and Cambridge that was formerly known as the establishment.