Trying to book somewhere in London for a special dinner is harder than you may think. With two weeks' notice sensibly timed tables at Kitty Fishers, Berners Tavern, City Social, Sky Garden - the brasserie and the restaurant - were all no-go's for a Friday night. We put ourselves in the safe hands of Mark Hix and settled on his New York style chicken and steak haven, Hix City. With neon signage from Tracey Emin and a definite contemporary cool, Hix is no stuffy City boy steake