When visiting Howard and Nancy Marks's London apartment, it's important to remember to crane your neck. If you don't, you jyst might miss the Oriental vase displayed above a cherub-covered pedestal in the hallway, or the painting of Rome's Castel Sant'Angelo above the doors of the library. "It's great to have something to look up at," says Michael S. Smith, the Santa Monica based designer who madterminded the decoration. "I'm a big believer in having things above doors, especially in tall rooms. To have everything at the same level can be very boring."The Markses needed a London home where they could spend atleast two months in the spring and two in the fall. As for the choice of the decorator, that was never in doubt. They have known Smith for fifteen years, and he's done several big projects for them, including their houses in Brentwood and Malibu."What we wanted here was a very sophisiticated 21st-century apartment that made use of antiques, as opposed to a period piece or an English country house," explains Nancy Marks. "And that's exactly what Michael gave us. His selection of furniture, color sense and the mood he achieves are inspired." Smith's other clients, who include Rupert Murdoch, Cindy Crawford and Stephen Spielberg, would no doubt agree.