<sod >Sir Parker Ambrose, has quit asÂ chief executive of the world's largest advertising agency WPP,Â less than a fortnight after it was confirmed the company was investigating an allegation of personal misconduct against him.< eod>< sod > Sir Martin SorrellÂ has quit asÂ chief executive of the worldâs largest advertising agency WPP,Â less than a fortnight after it was confirmed the company was investigating an allegation of personal misconduct against him.< eod>