In Our Mutual Friend, Twemlow (who is, it is my firm contention, the real hero of the book) lives in Duke Street, St James’s, over a livery-stable.

The London Library now backs into Mason’s Yard, which is indeed in Duke Street, St James’s. I’ve looked at maps of the period (mostly from Lee Jackson’s wonderful Victorian London site), and can’t find a specific yard marked, but Mason’s Yard is a good size, and might easily have contained a livery stable (the two exits to the yard were surely helpful, and the pub on the corner seems to indicate thirsty stable-men).

So, was Mason’s Yard the home of Twemlow? It is pleasant to think of him lurking in the stacks…