Another underground WWII tube bunker on the Northern Line

I had done any earlier (much earlier) post last year about the war storage capacity on the Northern Line.

Indeed the deep bunker is still visible from the road-side on Haverstock Hill.
http://474towin.blogspot.com/2007/11/its-hard-to-appreciate-that-tube-lines.html

What I hadn't appreciated is that there are in fact two and it was only when I was in Russell Nurseries that I spotted this second bunker visible. Wandering down the path next to Belsize Tube station there, bold as brass, is the entrance to the second bunker.

Now I really do have to do some serious research to understand the architecture and useage. And probably chase up the whole upkeep and graffiti etc here.