‘Belt and braces,’ he said and signed the forms.

I took the swab using the collection kit that I’d borrowed from Dominic who, I realised, had left the Boy Scout scale behind and was now verging on Batman levels of crazy preparedness.

When Joanne came down, Andy persuaded her to sign and swab and then she persuaded Hannah – who wouldn’t stop giggling. Then I mounted a detector at the front and back doors, or rather I watched as Andy neatly screwed them into position himself.

‘Just a precaution,’ I said.

‘I don’t like the idea of being watched,’ said Joanne.

‘This doesn’t detect you,’ I said. ‘It’s not a motion sensor.’

‘What does it do?’ asked Andy.

‘Hopefully,’ I said, ‘if certain conditions are met, it will stop working.’

I slipped out over the back fence and made my way down the backs of the village gardens to the Old Vicarage and the Laceys. On the basis that what the eyes don’t see the mouth can’t complain about, I planted a detector in their huge back garden before banging on their back door.

They met me in what estate agents call a reception room, what I would have called a living room and no doubt Nightingale called a parlour – unless it was a drawing room. In a country home this is not a sign of favour.

They didn’t offer me tea.

Derek made a big production of checking the consent forms while Victoria sat beside him on the sofa with her lips compressed down to a line and her hands jammed between her knees.

‘I really don’t see why this is necessary,’ he said.

‘A big case like this,’ I said, ‘even forensic evidence can get challenged. You know, as to collection and that sort of thing. Better to have two sets of samples – that’s why they’ve got me collecting it because I’m not from West Mercia Police and I’m going to send my samples to a lab in London. Separate force, separate samples, separate lab, separate chain of custody.’

Derek was nodding his understanding but Victoria was just staring at me, not angry or hostile, just impatient with one more aggravation she didn’t need right now – thank you very much. Still, like the Marstowes, they signed the consents and opened their mouths for the cheek swab.