The foreign secretary turned to gaze out over St James’s Park again. They watched the prime minister’s convoy turn out of the back of Downing Street and speed away down Horse Guards Parade.

‘As you will see from the file when I send it across,’ Sir Alan continued, ‘the original operation also pointed to a spy named Viper working somewhere in Whitehall, who had been helping their agent of influence.’

Meg Simpson turned back to face them. ‘I don’t recall you mentioning an agent in Whitehall.’

‘It was my husband,’ Kate said. ‘The Service has kept the matter top secret at my request.’

Simpson’s dark green eyes flicked from Sir Alan to Kate and back again. ‘It sounds to me as if the decision to close this file was rather too hasty.’

‘I don’t think we had any choice but to pursue the original intelligence to see where it led,’ Sir Alan said. ‘But, equally, given that we could find no proof of anything, I felt it was our duty to let the matter rest once the prime minister had taken office.’

‘I don’t understand. You just told me the operation to bug the yacht was a great success.’

Sir Alan could see immediately where this was going. He edged forward still further in his seat. ‘It yielded exactly the kind of intelligence we hoped it might.’

‘That the prime minister was working for the Russians?’

‘That our country’s security might have been compromised at the highest level.’

‘Might have been?’

‘That’s correct, yes.’

‘So you think the intelligence was accurate and that the man or woman they were originally referring to in the bugged conversation was our current prime minister?’

‘Yes.’

‘But you closed the file?’

Sir Alan wasn’t enjoying this. ‘Froze the file would be a better way of putting it. We pursued every available lead, but came to a series of dead ends. We had to take a view. He is our democratically elected leader. To put it simply, I felt we had to put up and shut up.’

‘So it doesn’t matter to you if our leader is working with – or for – the Russians? Given what is happening today and, if I’m going to be frank with you, his tendency to inaction on the subject, I should have thought it was all pretty relevant.’

‘Perhaps we should have done more to keep the investigation alive.’

‘It would seem so.’

‘But there was no proof and we had to bear in mind that the Russians might have been attempting to deliberately mislead us.’

‘Which would make the original operation less a success and more a catastrophic failure, I should have thought.’

Kate glanced at her superior, who seemed suddenly to be tying himself in intellectual knots in a very uncharacteristic manner. But, then, he looked dead tired.

‘And what do you think, Mrs Henderson?’

‘About which aspect in particular?’

‘Is the prime minister a Russian spy?’

‘Yes.’ Meg Simpson gazed at her steadily. ‘I mean, I can add caveats about what exactly we mean by that term. But if you’re asking me whether he has been compromised in some way by the Russians, then, yes, I think he has.’