She had the same Aberdeen accent as her husband. They could have been born in the same street.

‘Who’s this?’

‘New man. DC Wolfe.’

‘Hello, hen,’ Mrs Mallory said, her face breaking into a broad smile. ‘Come on in.’

She brought us tea and biscuits, and I began to feel better with sugar and caffeine inside me. Mallory bolted his tea quickly and disappeared, shoving in a ginger nut.

‘Five minutes,’ he told me.

The Westie followed him, panting with pleasure.

Their home was a small, neat maisonette. The photographs on the mantelpiece and bookshelves, their colours fading now, showed the Mallorys’ old life under some tropical sun. The pair of them, fifteen, twenty, twenty-five years younger, raising glasses and smiling shyly at some café table. And Mallory, his hair already gone at thirty, grinning in shorts, short-sleeve shirt and a black peaked cap, two Asian men in the same uniform grinning either side of him. And the Mallorys smiling again for the camera with a city built by a harbour behind and far below them.

‘Hong Kong,’ Mrs Mallory said. ‘My husband was in the Royal Hong Kong Police Force for fifteen years. “We serve with pride and care”. Maybe he told you.’

‘No, ma’am.’

She laughed. ‘You can call me Margaret.’

‘DCI Mallory hasn’t told me anything, ma’am – Margaret.’

‘We came back in 1997. After the changeover. When it lost the royal bit. The British went home and so did we. Bit of a shock to the system. We miss it. Although of course that old Hong Kong is not there any more.’

The photographs ran the course of two lifetimes. But I could see no pictures of children.

‘And do you have a family?’ Mrs Mallory asked me.

‘I have a little girl,’ I said. ‘My parents are long gone. No brothers, no sisters.’ A beat. ‘I lost my wife.’

She waited for more, but there wasn’t any more.

‘I see,’ she said.

‘I have my daughter. Scout. She’s five.’ I stirred my tea. ‘But there’s just me and her.’

Mrs Mallory nodded. ‘Then you have a family,’ she said, and she had me for life.

We crossed the river as the sun set, the groups of tourists on the bridge taking photographs of the Palace of Westminster, Big Ben and Westminster Abbey, all majestic in the dying light, as the London commuters hurried home, not even noticing the everyday magic of this place.

‘And now you’ve met my wife,’ Mallory said. ‘Take the Lambeth Road on the far side of the bridge.’

‘Sir.’

I drove south until two giant cannon reared out of the twilight. They sat before a domed building flying the Union Jack.

‘We’re going to the Imperial War Museum?’

‘It was once the Bethlem Royal Hospital,’ Mallory said. ‘Bedlam. The lunatic asylum. Did you know that? You can park round the back in St George’s Road. We’re not going through the front door.’