Lal shrugged. ‘Mehmet let me have it. Akash is a friend of his. My sister got me the placement at Mount Browne.’

‘So you thought it was a good idea,’ Claire said, ‘to try and get your own back on the police who were investigating you?’

‘No, that wasn’t it at all. I was just hiding. It was Mehmet’s idea. He got it from Osama bin Laden, hiding in plain view. You know how he was caught in 2011, living in the same town as the headquarters of Pakistan’s security forces?’

‘We know,’ Gillard said. ‘So how long had you been working in the kitchens at Mount Browne?’

He shrugged. ‘Four nights. The idea was just to keep my head down for a while. I wanted you off my back.’

‘So why the poison?’ Claire asked.

‘What poison?’

For the next half an hour Lal denied every accusation put to him, except possession of the drugs found in his flat. ‘There wasn’t even enough to convict me for dealing,’ he said.

‘I beg to differ, Mr Lal,’ Gillard said.

‘Harry is going to be very disappointed in you,’ Claire said.

He blinked angrily. ‘Bullshit. Harry trusts me, Harry has always trusted me.’

‘You killed his mother,’ Gillard said. ‘You created a false alibi, to convince him, more than anyone else, that you had nothing to do with it.’

‘You don’t have to answer,’ Shah said gently.

Lal locked gazes with Gillard. ‘You don’t have anything on me.’

‘Really? We found some highly incriminating evidence with your downstairs neighbour Mrs Clark.’

The suspect’s face tightened, and doubt showed in his eyes. Claire opened a folder and passed across a sheaf of photographs.

‘We found this bag concealed in a cupboard in her kitchen.’ She pointed at one picture, and then another. ‘These are two 170g bags of cocaine. Your fingerprints are all over them. And this,’ she said, pointing to a third image ‘This is Mrs Roy’s Bottega Veneta purse. Lost at the murder scene. How come you have it?’

Lal seemed to sag under this blow.

‘You kept it to make it look like a mugging, but you didn’t think we’d check with the neighbours, did you?’ Gillard said. ‘But ever since I found you did your Internet search on thallium at Morag Fairburn’s house, it’s been clear to me that you have no compunction about leaving your dirty linen on other people’s doorsteps, for them to take the blame.’

Zayan Lal was remanded to Coldingley Prison near Woking for a week, and Gillard left him to fester with his own thoughts for two days before coming back for a fresh interview. It was a carefully planned psychological strategy to get a confession. They had allowed Harry Roy to visit Zayan on Wednesday afternoon, and turned up incognito in advance of that meeting.

The detectives knew that nothing they could do would soften up the prisoner more than the disappointment in the eyes of his lover. From the prison’s security office the detectives watched the CCTV from the visiting room, though they could not hear what was said. Zayan was almost prostrate on the table in front of Harry, clearly beseeching him to believe his story. Harry meanwhile had his eyes closed for most of the encounter, leaning back as if he didn’t want to touch, and shaking his head in disbelief. At the end of the allotted time, Zayan held his arms out to Harry, who shrugged him off and lurched towards the exit. A custody officer had to restrain the prisoner, who was clearly calling out to his former lover.