I found Richmond Buildings and Richmond Mews on this old map, just to the left of the bottom left corner of the big red "H". Richmond Buildings is directly off Dean St and then Richmond Mews runs south from Richmond Buildings and parallel with Dean St.
http://www.oldlondonmaps.com/stanfordpages/westminster03b.htmlI believe it is the same today.
So the 'tablet' mentioned would be on the the one remaining end of Richmond Buildings facing Dean St?
I have just had a look on google maps street view and note that there is only an old building on one side of the entrance to Richmond buildings - it is No 80.

The other side is an ugly modern structure. I have amended my previous post to correct my mistakes.

And yes, I believe this is possibly what remains of the south side of Richmond Buildings facing Dean St.
I have also been thinking about Nathaniel's mother Mary who we cannot find in the 1841 census, though when she marries she says she is from Dean St. I wonder if she lived in a part of Richmond Buildings facing Dean St? We know Matthew Ward was living in Richmond Buildings in the 1841 census and when he married Mary his address was Richmond Buildings. I wonder if Mary preferred to say her address was Dean St to appear more respectable and so it didn't seem as though they were living together?
