A teenage boy on the pavement opposite filmed the whole thing on his mobile phone. He was in Year Ten at Haddon Park and I wondered how long it would be before his video appeared on Facebook. They wouldn’t use handcuffs if he wasn’t guilty as sin.

Redford stayed with me, a silent presence, as I was booked in with the custody sergeant at Kilburn Police Station and told to empty my pockets. It all went into a clear plastic bag: two mobile phones – both mine and the secret phone that I’d found in Mel’s handbag – wallet, keys, a ballpoint pen and loose change, before I signed at the bottom of a form and it was sealed for storage in a custody locker behind the counter.

The custody sergeant slid two more forms across the desk. The first was basic information about my name, address and any medical conditions. The second form requested consent for the police to take fingerprints and DNA samples from me. I had done nothing wrong, and yet . . . there was something about this that felt like crossing another line. In the system, forever, my unique set of numbers stored in a computer waiting for the day when I stepped out of line. It felt like giving up a small piece of freedom.

Being arrested was starting to feel very real indeed.

I signed the form and the sergeant fingerprinted me, carefully and methodically. He led me into a side room where another officer took a sample of DNA, a swab in my cheek for saliva. Left cheek, right cheek, sealed tight inside a labelled tube, job done. Then another one: identical procedure, identical routine.

‘Why do you need to do it twice?’ I asked the policeman, a young officer who looked barely old enough to drive.

‘One’s the control sample, one’s the secondary,’ he said. ‘We test them both to make sure they’re consistent with each other before we put them into the database. It’s like a back up.’

‘In case of mistakes?’

He shook his head.

‘It’s about our procedures being watertight. A match is a match: DNA doesn’t lie.’ He glanced up at me. ‘Unlike people.’

Redford swiped us through the reinforced door that led into the back of the station, showing me into a small interview room, smaller and shabbier than the one I’d been in yesterday. She disappeared. There was no offer of tea or coffee this time.

Forty minutes passed.

Redford was the first to return, a laptop under her arm, her face as blank and expressionless as marble. She was followed by Peter Larssen, who sat down next to me in a cloud of aftershave. Naylor was last to arrive, his tie pulled down below an open top button, folder in hand, hangdog expression firmly in place.