‘She did,’ Gillard said. ‘But that’s not the end of the story.’

Yaz Quoroshi returned with his iPad in hand. ‘Craig, I’ve just been checking, and I think we can save some time on the tests. We can do them ourselves this afternoon. I’ve arranged with my wife, I think you met Jenny, who is a research chemist, to borrow a lab at Guildford University. It’ll be a good training exercise, and it will only take a couple of hours.’

Later that afternoon Gillard and DI Claire Mulholland were sitting with the three trainees in a large lab at the university. They were all wearing white coats and safety glasses. Jenny Quoroshi bustled around in the lab labelling everything. Each of the foodstuffs had been numbered and marked up, and a liquefied sample of each put into a correspondingly numbered test tube. There were almost 200 of them in a rack. Mrs Quoroshi had already mixed together bismuth nitrate, sodium iodide and nitric acid to make a reagent, and was now using a teat pipette to put a drop in each of the tubes.

‘Now this will give a red precipitate if any thallium is present,’ she said. ‘It will only take a few minutes. It is possible for there to be false positives in this test but at least we will then only have a handful of samples to send on for spectroscopy. That should save the good old British taxpayer a good deal of money.’

‘Always happy about that,’ Gillard said, smiling at Yaz.

The two detectives and the trainees watched in silence as each of the samples received a drop of the reagent. They waited. And waited. After ten minutes not one of the foodstuffs or medicines had tested positive.

‘That’s a little bit of a disappointment,’ said Yaz.

Gillard got up and walked up to one of the boxes from the flat, packed with all the contents of Mrs Roy’s hypochondriac cupboards. ‘Did you test the toothpaste?’ he asked, picking up a tube.

Yaz nodded. ‘We did that and the mouthwash.’

‘What if the poisoner ditched whatever it was?’ Kirsty said.

Gillard looked up at her as she continued. ‘I mean, if he or she could get in to plant something poisonous, then it would be possible to come back a second time and remove it.’

‘That’s true, of course,’ he replied. His phone began to ring, and before he answered it he said: ‘Just make sure you test everything.’

The call was from DC Colin Hodges about Gabby overhearing a conversation at Harry Roy’s flat. ‘The number plate on her photograph is of a vehicle belonging to Deepak Tripathi.’

‘Ah, Prisha Roy’s ex-husband. Thank you for that, Colin.’ Gillard hung up. It was the first glimpse of the Roy family’s secrets, and a fascinating one. If you learned to keep it zipped up. So Deepak was castigating Harry for something. Whatever it was, could it be anything to do with the murder of his mother? Then he realised that Deepak was the one person closely associated with the family that he hadn’t had the opportunity to take a DNA swab from. He’d remedy that as soon as possible.