Bertolt Brecht took The Threepenny Opera  from an 18th-century script by John Gay and relocated it to Victorian London. This National Theatre version wants to straddle the contemporary and the antique. Mack the Knife, an Afghan war veteran who murders strangers, contracts a bigamous marriage with Polly Peachum, the daughter of a cross-dressing mastermind who runs begging gangs across east London.