Time Out says
An informal deli and café at the Amba Hotel Marble Arch. It's located just off the hotel lobby, and is open 24 hours a day for hotel guests, so if you're staying the night and fancy a midnight munch you can get stuck right in.
Throughout the day it's open to the public, with simple lunch dishes (think sandwiches, snacks etc.) available to eat in or to take away. The evening is open for drinks, with hotel guests and passersby popping in for a pre- or post-dinner snifter or two. Afternoon tea is also available.