Sir Martin Sorrell’s resignation has raised the question of whether his successor will be able to stop a breakup of the world’s largest advertising group. Some analysts believe that there is more value in breaking up the sprawling empire – WPP employs more than 200,000 staff in 400 separate ad businesses in more than 3,000 offices in 112 countries – particularly as its stock market value has plummeted more than a third in the past year