By James Davey LONDON (Reuters) – On Wednesday, Steve Rowe will step down as chief executive of Marks & Spencer after six years, ending a 39 year career at the British retailer. He will be succeeded in one of the most high profile jobs in British business by M&S’s food boss and joint chief operating officer Stuart Machin, who will become chief executive and take on responsibility for day-to-day leadership of the business and executive committee. An unconventional leadership structure will see Machin backed up by co-chief executive Katie Bickerstaffe and finance and strategy chief Eoin Tonge. H…