London-knowlegeable people! Help, please!

(For work, believe it or not) I'm trying to find the name of a triangular building in London (to get the dimensions, which is easier once we know what it is). It's near the Charing Cross tube station, on Strand Street on the southeast, on William IV Street on the north, and just behind St. Martin-in-the-Fields church. The descriptor of a photo of the area imply that it might be Coutts' Bank, or partly so, but perhaps not. Can anyone help?

ETA 1300 Many thanks to everyone who helped. The researcher assures me he now has enough info to be able to find it (and sends his thanks as well).