Clement • Link
Desinged by Inigo Jones and completed in 1627, it hosted Roman Catholic services for the next three queens, and served others during the interregnum and when there were no Royal Catholics in power. Today it is perhaps Jones' least altered surviving work.
From HRH Margaret's memorial site, with a picture:
Photos from a walking tour:
Further history in this first page of an article by Simon Bradley in the journal, Architectural History, Vol. 44: