This is a very narrow staircase in the Jewel Tower, London, It was built in approximately 1365 to house the treasures of Edward III. It now contains an exhibition called "Parliament Past and Present", as it is across the road from the Houses of parliament, Westminister.
Thanks Viv & Cees,
At first glance, it looks like the stairs are going down, but if you see on the right below the window they are in fact going up! This is one of the best spiral staircases I have seen, & I was lucky it was not full of people walking up & down!