‘It had to be tiny, otherwise Mrs Roy would have spotted it. And because the roll-on was normally stored vertically it wouldn’t have leaked with a hole two thirds of the way up. Until we took it as evidence. My guess is that she didn’t take thallium into her body through food or medicine, but through her armpits.’

Hodges stared at him in horror.

‘This is not accidental,’ Gillard said. ‘We are dealing with a devious and knowledgeable killer.’ And, he realised, it’s definitely not Jason Waddington.

Luckily for Gillard, Yaz Quoroshi was at home that Sunday afternoon. The detective could hear sport on the TV in the background as he described his finding to the head of crime scene investigation.

‘I’m going to ask you a big favour, Yaz. And a favour from Jenny too. Is it at all possible to get back to the lab this afternoon and run the test on this deodorant? It’s just the one item.’

Quoroshi sucked in a big breath. ‘Yeah, I think we could do that. Arsenal are losing anyway Let me just ask.’

Forty minutes later all three of them were in the lab at Guildford University, wearing white coats, eye protectors and thick rubber gloves. Before them was a plastic evidence bag containing the roll-on. Jenny Quoroshi carefully picked it up, unscrewed the cap and inverted it. She then used a cotton bud to move the ball, so that a smear of liquid emerged. She wiped the bud in it, soaking it as much as she could, then dropped the bud into a test tube on a stand. From a locked cupboard she removed the surplus reagent which she had mixed for the previous test. The mixture of bismuth nitrate, sodium iodide and nitric acid would turn red in the presence of thallium. Using a teat pipette, she put a single drop into the tube.

It was only a couple of minutes before they noticed a dark red precipitate begin to form in the bottom of the tube.

‘That’s it,’ Jenny Quoroshi said. ‘Positive for thallium. We need to get it confirmed with a spectrophotometry test. Can do it here, but it will take a day or so.’

‘I’m looking forward to showing this to Dr David Delahaye,’ Gillard said. ‘He was leaning towards an accidental exposure to thallium. But there can be nothing accidental about this.’

Chapter 13

The sound of the quad bike was gradually driving Prisha mad. She looked out of the kitchen window into the paddock, where her eleven-year-old daughter Indigo was driving round and round and round on a bumpy circuit through the orchard. Perhaps the neighbours did have a point about the racket. And maybe it was a mistake to have bought her the damn thing, but even if there was only a slight chance it was worth taking.

Yesterday’s rain, and her daughter’s incessant laps, had turned half the paddock into a muddy quagmire. Prisha drained her coffee, made her way into the cloakroom and – shoving aside Indigo’s skateboard, football gear, and hockey stick – dug out her own wellingtons. Donning them and a rain jacket, she went out into the cold, crossed the thirty yards of formal garden and arrived at the hardstanding by the garage. On Indigo’s next pass this side of the spinney, Prisha hailed the mud-spattered vehicle, whose original bright yellow bodywork was barely visible. Indigo waved to her mother in acknowledgement, and standing up, accelerated up a hillock, spraying earth in all directions. She then executed a tight turn, and raced the quad at her mother. Prisha stood her ground, annoyed and encouraged in almost equal measure. Indigo had inherited plenty of her own fierce independence, and with it an almost frightening intelligence. She had realised that when they discovered two years ago that Indigo was selling her lunches of EoS past-date products to fellow pupils, and even members of staff. But this quad bike had unzipped in her a layer of almost masculine aggression. The child slammed on the brakes just in time, and swerved to a halt on the gravel.