Windrow’s mouth worked, and I suspected he was wondering whether it was medically advisable for him to have another piece of nicotine gum.

You may have chosen the wrong moment to quit smoking, I thought, but I didn’t say, because you don’t – not to chief inspectors.

‘I’m assuming,’ he said at last, ‘that you have a line of inquiry you’d like to follow.’

‘We take DNA samples from both girls and their parents and then we test to see if they are who we think they are,’ I said.

‘And what do we tell them we’re doing it for?’

‘For elimination purposes,’ I said.

‘I know the Met has a reputation for being a bit free and easy with the facts,’ said Windrow. ‘But you do realise that we’re talking about the victims and the victims’ families here, and that we’re operating with the full sodding media pack camped outside our door. They may not know what the story is, but they can smell there’s a story. Not to mention that Sharon bloody Pike has an inside line to the Lacey family. Do you really think that, given all this, it would be a good idea to obtain DNA samples under false pretences?’

‘Sir . . .’ I said as neutrally as possible in the time honoured tradition of interrupting your senior officer when he’s being rhetorical.

‘If she is a . . . “substitute”,’ said Windrow, ‘what’s the worst case scenario?’

‘If she’s been swapped, then Nicole Lacey is still being held by whoever made the change,’ I said. ‘In which case, this is still a live kidnapping inquiry.’

And if there’s a case review and it turns up that we didn’t do our due diligence, then it wasn’t going to be me answering the tricky questions, was it?

Windrow nodded.

‘I want you to get the necessary authorisation from your governor and run this as an official Falcon line of inquiry,’ he said. That covered him from any case review, and it also gave him plausible deniability should it blow up in the press. ‘I also want you to be the one to approach the families and get the samples.’

I said I was fully prepared to do that.

‘And don’t discuss this with anyone but me and your boss – understood?’

I understood. He didn’t want any leaks to the press – or at least, in the event of there being a leak, he wanted to make sure it wasn’t traced back to the MIU or, bonus, the West Mercia Police. Mind you, this sort of compartmentalisation suited Nightingale down to the ground – someone had once told him in 1939 that loose lips sink ships and he obviously hadn’t seen any reason to change just because the war was over.

‘Yes, sir,’ I said and rushed off to obey.