The novella 'What Abigail Did That Summer' is pretty explicit about the location of The Folly.

> ...sits on the south side of Russell Square on the corner of Bedford Place.

As the west side of Bedford Place contains a glass and metal office block that would make this The Folly.

50 Russell Sq
https://maps.app.goo.gl/XdyAU5nYcXoXgqfM6