Morgan McSweeney, the Prime Minister’s Chief of Staff, does not immediately display the demeanour of a disruptor. He speaks softly, picks his phrases with care, and cultivates an unassuming image. But underneath the sober blue suit are the scars of a streetfighter. As a young man, McSweeney came to political maturity fighting the hard left in Lambeth and the far right in Dagenham, winning back working-class voters to a Labour party that had forgotten its roots.