We shook hands and walked out to the Terrace Pavilion, where there were white tables and chairs and people drinking tea. We stood with our backs to them all, the Palace of Westminster rising above us, and the Thames flowing far below.

‘My condolences,’ I said.

‘Thank you.’

‘You were with Guy Philips when he died.’

‘Yes. I spent the last night in his room.’

‘Did he say anything? I’m sorry but I have to ask you.’

Ben King stared at the river without seeing it.

‘It was four in the morning. He was sleeping. I was sleeping. I awoke because the machine that monitored his heart rate was making a different sound. I called the nurse. She was sleeping too. When she came to the room she immediately called the doctor. But it was too late.’ At last he looked at me. ‘Guy died in his sleep. I know what you’re asking. But he never woke, he never said anything about who attacked him and he passed away peacefully. And I am profoundly grateful for that small mercy, detective.’

‘And you were with him every night he was in the hospital?’

‘Yes. Guy never married. His parents were dead. My brother and I – all of his friends – were what he had instead of family.’

‘I think he saw his killer’s face. I’m certain of it.’

‘He didn’t say a word.’

A young blonde woman was standing in the entrance to the Terrace Pavilion, trying to attract King’s attention. He raised his hand to show that he understood. But his manners were too good to rush me.

‘There’s something else,’ I said. ‘You – and your brother, and Mr Khan – you’re all going to be issued with Osman Warnings.’

‘Osman Warnings?’

‘If the police have reason to believe that someone is at risk of being murdered or seriously injured we issue an Osman Warning – it’s both an official warning and an offer of police protection.’

King almost smiled. ‘It sounds rather like a way of the police trying to protect themselves from future accusations of negligence.’

‘There’s an element of that,’ I agreed. ‘My superiors will be writing to you to offer protection and to give advice on the steps you can take to ensure your safety.’

‘Is that really necessary? Perhaps for Salman – who, I understand, is now too afraid to leave his house. But I’m surrounded by high security in the House. And Ned is out of the country and has the entire British Army watching over him. I think he’s more at risk from the Taliban.’

‘We have a duty of care to warn people when they are in mortal danger,’ I said. ‘On Hugo Buck’s desk there was a photograph of seven boys at Potter’s Field. You probably know the photograph.’

‘Yes, I know it.’

‘Now only four of you are still alive.’

He was watching the river again.