The origins of producer Cameron Mackintosh’s brand new West End tribute to composer-lyricist Stephen Sondheim can be traced to a trip up the M4 in 1976. He had taken the journey to view the new revue Side By Side By Sondheim at the Cleo Laine/Johnny Dankworth-owned The Stables in Wavendon, but subsequently realised that Wavendon is, in fact, off the M1, and he’d been heading in the wrong direction.