It is one of several large photo/compositions, collectively called "Hidden", and will be on display in the foyer of the Museum of London until 6 April. I sincerely hope that the collection will remain on the website indefinitely.
Here is the image of Mary, and here the Museum's general explanation of what it calls "tableaux vivant":
"Hidden" engages with a different historical narrative involving dissenters, revolutionaries and radicals. Red Saunders combines his photographic practice with cultural, artistic, musical, and political activism. He said, "I recreate important moments in the long struggle of working people for democracy and social justice. History has been dominated by kings, queens, war and ‘great men’."