Mystery solved

Maurice1948 asked me if I could identify an object he had photographed in London recently - the rather striking triangular structure in my blip. It turns out that it is the top of the Emergency Exit stairs from the southbound platform of Thameslink at City Thameslink station.  Very impressive it is too - side view of the black marble cladding in the extra. It's at the top of Fleet Place which is a fairly new development of mainly lawyers' glass skyscrapers in the shadow of the Old Bailey.