IN THE HEART of the City, at the corner of Cannon Street and King William Street, stands a Mercedes-Benz dealership. In pride of place in the window is a sleek, silver sports car, the SL 55 AMG, priced at £94,995. It is midday, the start of the lunchtime rush, when traders and City bigshots can leave their desks and do some shopping. But the showroom, which was only opened in March, is devoid of potential customers.