As I live local to the NPL I thought I'd do a walk around with a pair of Mark 1 eyeballs to see what I could see.
I did not see anything that fitted exactly which does not mean it is not somewhere in there as there are many buildings all hidden one behind the other and the perimeter consists of high walls and fences often with trees and bushes blocking the view. I did see a couple of smaller older buildings that had similar brickwork and windows, though the frames are painted white, but the entrances did not match.
If you live anywhere nearby you might be interested to know that Bushy House, the original home of the NPL, is having an open house on Saturday 17th September which might open up other views of the main site or you might find someone from the NPL that recognises the scene.
HTH,
Paul