Celia finished her call and came over. She was an athletic woman with short, dark hair and bright blue eyes, dressed in yoga leggings and a thick white polo-neck with a gold necklace at her throat. ‘Kate, how nice to see you again.’ She turned to the others, who introduced themselves. Celia shook Callum’s hand, then faced Julie. ‘You’re the young woman Ian had an affair with.’

It was said without noticeable bitterness or rancour, but Julie still looked as if she had been slapped, which perhaps she had, the blow all the more powerful for being delivered in such measured tones. Celia smiled at her, then retreated to the sofa opposite them, tucking her legs back as she leant forward to offer them all coffee.

‘How’s business?’ Kate asked. Celia ran a mail-order and online sleepwear empire, which had once been one of Ian’s boastful riffs. He’d set great store by having such a clever, successful wife.

‘Booming.’ Celia smiled again. ‘I seem to be rather better at business than marriage.’ She glanced at Julie, who was staring into her coffee, as if it was about to offer an answer to the meaning of life. ‘But something tells me you didn’t drive all this way to talk about my commercial activities. Perhaps you could start by telling me why your colleague here suggested secrecy was of such critical importance?’ She was pointing at Callum.

‘We’ve been asked by the prime minister to conduct an investigation outside the normal procedures within the intelligence services.’

‘Into Ian?’

‘Into the possibility we have a traitor at the heart of SIS.’ Kate shrugged. ‘You always struck me as a decent and highly intelligent woman, so I’m doing you the courtesy of telling you the truth and trusting that you’ll see the seriousness of the situation.’

Even as she spoke, Kate wondered about her breakfast with Ian and his implicit warning. Had Celia tipped off her former husband about this meeting, or was it just coincidence?

‘All right. Since you put it like that, I accept. I’ll tell no one what transpires in this conversation,’ Celia said. ‘I don’t have much of an appetite for the cloak-and-dagger world, for reasons you can probably guess at, so you can do me the favour of making this quick.’

Kate gazed at the woman opposite her, thinking again of the mystery men remained to her. Who would want to cheat on a wife like Celia? ‘Maybe the easiest thing is if I share with you a working theory.’

‘Go on.’

‘We know we have a double agent working at the heart of SIS. We know that he or she has been in place for a long time, almost certainly from the mid-nineties. And we know that they have spent much of their career involved with Russia. But, as you will understand, whoever it is has been clever enough to cover their tracks.’

‘Well, we can probably all agree that Ian is clever.’ Celia smiled at Julie again. She was getting her money’s worth here, that was for sure, and in the most elegant way imaginable.