Leinster Gardens in Bayswater is a long white terrace of five-story Victorian houses with Corinthian columns, heavy front doors and iron balconies. Numbers 23 and 24 fit in perfectly with the rest of it, which took some doing, because there’s nothing behind them.

The are clues giving it away if you know where to look. The windows are painted on, they’re just flat grey rectangles that never catch the light the way the real ones do. There are no curtains and the doors have no mailboxes or keyholes. Everything above the front step is a five-foot-thick shell of brick and stucco, propped up from behind with steel girders, and past that is a straight drop into an open railway cutting.

The Metropolitan Railway opened the world’s first underground line in 1863, running between Paddington and........

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