It's from one of the redundant carriages now doing duty as artists' workshops and offices up a narrow spiral staircase on top of an old railway arch housing a massage parlour.
The collection of carriages is known as "Village Underground" (well, it is and it isn't, geddit?), and was welcoming visitors last weekend as part of the Open House event (I didn't really set out to see much this year, but this was on my route somewhere else, so I thought I might as well take a look).
The carriages mostly seem to house about half a dozen people each, with desks and all the usual office paraphernalia replacing most of the seats. I did ask one of the occupants if they found it got over hot in the sun, but apparently it's enough to leave a door open.
One of them, however, was being used for dress or costume-making, and was thickly festooned with cardboard pattern cutouts, swathes of fabric, and people ironing and pressing away - shades of the Shoreditch sweatshops of a century ago, perhaps.
Gingerly making my way down the stairs, who should be waiting to go up but Joan Bakewell - clearly this was the place to see. From the street, you can see why people want to find out what it's all about: