‘But they communicate privately, don’t they, as old friends? Not through Office comms to which we have access. And without a warrant we couldn’t get across Gareth’s private calls or texts or emails or WhatsApps or whatever they do. And to get a warrant on Gareth in this country we’d have to go through MI5 and the home secretary and I can’t see we’d get it. There’s not enough to make a national security case against him, even if we wanted to.’

‘We do it from the other end. Timber Wolf. He’s a foreigner living abroad in whom we have a legitimate intelligence interest, if only in checking that he’s playing straight with us. I can put in a request to Cheltenham and they can access Timber Wolf’s comms – or his comms record – with a few clicks of a mouse.’

‘But could they make a message from me appear to come from Gareth? And divert any reply to me?’

She shook her head. ‘I know I’m a digital dinosaur but you’re positively pre-Cambrian. That’s bacon and eggs for Cheltenham. If they can fix your laptop so that it explodes when you switch on I’m sure they can redirect the odd text or two.’

‘They can do that, can they? Blow it up?’

‘So I’ve heard. Ask your opposite number.’

‘Gareth will be pretty upset when he finds out I’ve seen Timber Wolf.’

‘Don’t wait for that. Tell him yourself, after the event. If it proves – as I suspect it will – that he’s been a naughty boy, exaggerating intelligence, then he’s no cause for complaint. If it proves him innocent then you simply say you had to do it so that you could reassure Whitehall with a clear conscience. And that will do no harm at all to his chiefly ambitions.’

‘And meanwhile I say nothing about our doubts to Robin Woodstock but simply sound him out for retrospective clearance on the grounds that Timber Wolf is effectively a paid agent?’

‘Got it in one, professor. Now, can we get out of here, please? I can feel my cholesterol going up just through looking at the place.’

In the Foreign Office Robin Woodstock was brisk. ‘Sorry to rush. Morning meeting’s been brought forward. The secretary of state’s diary is a living exemplification of the second law of thermodynamics. The uncertainty principle, if I’ve got that right.’ He grinned. ‘Will ten minutes do or d’you want to rearrange?’

‘We can do it in five.’ That suited Charles. Less time meant less chance of the discussion widening to the point where he would either have to admit his doubts or mislead. He explained briefly.