She caught me looking and tilted her head in amused recognition before turning and walking away on unexpectedly sensible flat shoes.

I knew that walk, a brisk, business-like walk. A walk to cover the distance quickly without looking hurried or worried.

I keyed my Airwave.

‘I’ve just seen Lesley at the Arch,’ I said.

‘Are you certain?’ asked Nightingale.

‘The face is different but it’s definitely Lesley,’ I said.

‘Is it a mask?’ asked Nightingale.

‘No,’ I said. ‘Not a mask.’

‘Did she see you?’

‘Yes.’

‘Arrest her,’ said Nightingale. ‘Now.’

I was wearing trainers – you can run really quietly in trainers if you have to. It’s all in the way you roll your feet. I took off as fast as I could, straight for her back. I knew sooner or later she was going to check on my position, but I was counting on her Lesley-esque sense of drama to hold the moment longer than she should. I was three metres short when a white guy in a navy suit jacket over a black Metallica T-shirt theatrically jumped out of my way and yelled, ‘Look out.’

The woman who walked like Lesley turned and I saw the look of surprise on her face. Then I stumbled as the pale skin of her face rippled and her features changed. It started at the bridge of her nose, the skin bunching up and then flopping down horribly, like the wings of a manta or the shroud of a squid. Then suddenly it was Lesley’s face again – or rather the smooth pink version of it I’d seen in the Harrods Jazz Café. I was so shocked that I barely registered the raised hand and the shimmer in the air that signalled pain inbound. I forced myself to lengthen my stumble into a fall and a clumsy roll, taking the impact on my shoulders, as something shot through the space I would have been in, with a noise like my mum beating a carpet.

As I climbed to my feet and dodged right on general principles, I heard yells behind me. I had the Airwave off by then and didn’t dare to turn it back it on to alert Nightingale to potential collateral. I had to trust to his professional instincts and hope that Lesley wasn’t flinging anything too lethal around.

She knew better than to escalate in public in central London – not unless you wanted to take a run up that short ladder that ended with making a personal relationship with the Special Air Service.

I ran through the arch, making my appearance as abrupt as I could, and spotted Lesley heading off towards the Wellington Memorial. She was walking briskly rather than running – hoping, I assumed, to avoid drawing attention to herself.