A second mahogany door led to a narrow marble stair, lit by cubic wall lights, which took them on to a small lower landing, where a plain painted door led to the underground car park. Wilson opened it with two keys, a Chubb and a Yale, then flicked a switch. The floodlit area was almost as long as the street itself, full of millions of pounds’ worth of Ferrari, Audi, Bentley, Jaguar and BMW. At twenty-foot intervals along the back wall were doors like the one through which they had just come: inner entrances to each of the houses of Kentigern Gardens. The electric garage doors leading from Serf’s Way were close by number 18, outlined by silvery daylight.

Robin wondered what the silent men beside her were thinking. Was Wilson used to the extraordinary lives of the people who lived here; used to underground car parks and swimming pools and Ferraris? And was Strike thinking (as she was) that this long row of doors represented possibilities she had not once considered: chances of secret, hidden scurrying between neighbours, and of hiding and departing in as many ways as there were houses in the street? But then she noticed the numerous black snouts pointing from regular spots on the shadowy upper walls, feeding footage back to countless monitors. Was it possible that none of them had been watched that night?

‘OK,’ said Strike, and Wilson led them back onto the marble staircase, and locked up the car park door behind them.

Down another short flight of stairs, the smell of chlorine became stronger with every step, until Wilson opened a door at the bottom and they were assailed by a wave of warm, damp, chemically laden air.

‘This is the door that wasn’t locked that night?’ Strike asked Wilson, who nodded as he pressed another switch, and light blazed.

They had walked on to the broad marble rim of the pool, which was shielded by a thick plastic cover. The opposite wall was, again, mirrored; Robin saw the three of them standing there, incongruous in full dress against a mural of tropical plants and fluttering butterflies that extended up over the ceiling. The pool was around fifteen metres long, and at the far end was a hexagonal jacuzzi, beyond which were three changing cubicles, fronted by lockable doors.

‘No cameras here?’ asked Strike, looking around, and Wilson shook his head.

Robin could feel sweat prickling on the back of her neck and under her arms. It was oppressive in the pool area, and she was pleased to climb the stairs ahead of the two men, back to the lobby, which in comparison was pleasant and airy. A petite young blonde had appeared in their absence, wearing a pink overall, jeans and a T-shirt, and carrying a plastic bucket full of cleaning implements.

‘Derrick,’ she said in heavily accented English, when the security guard emerged from downstairs. ‘I neet key for two.’

‘This is Lechsinka,’ said Wilson. ‘The cleaner.’