Kate didn’t break her stride. She went to a café called Pimlico Fresh at the end of Wilton Road and ordered a latte. She took it to a vacant seat outside and looked up and down the street to see if she could make out any more of her watchers.

She wasn’t in much doubt as to who they were or what they were doing, but she wanted a few moments to think about it. She had toyed with the idea that the men and women following her might be Russians, but she had dismissed it. Unless Moscow’s operational teams had made epic strides in their ability to assimilate convincingly into a London scene, it was not conceivable. Besides, why on earth would the Russians want to follow her to lunch?

Kate took out her phone and called Suzy. Her deputy answered straight away. ‘When were you thinking of telling me you’ve put a tail on me?’

‘I haven’t done anything, Kate.’

‘All right, let me rephrase the question. When were you thinking of telling me that your friends at the Security Service have put a tail on me?’

‘I think I made clear they are not really my friends.’

‘Could you at least deny it?’

‘I told you they’d opened an investigation. The plain fact is you almost certainly have a mole. It’s their job to find him or her.’

‘I thought C had expressly forbidden this?’

‘The DG made a courtesy call. C tried to claim that, if there was a problem, it was an internal matter and any investigation would breach operational security, but the integrity of all government agencies is MI5’s business, as you well know. I did inform them of my questions about Viper, but it’s out of my hands now. It’s out of all of our hands, to be honest.’

‘Did they put their most incompetent team on me, just so I would know they were watching?’

‘I doubt it. They don’t really think like that.’

‘What do they hope to achieve by trailing me to lunch?’

Suzy sighed. ‘This isn’t really going anywhere. I’ll see you when you get back.’ She ended the call and Kate nursed the rest of her coffee in cold fury. Her mood hadn’t lifted by the time she arrived at lunch with Sir Alan.

He was in the corner booth, which she knew to be his regular, opposite a sign for ‘Rue Mozart’. It was a cosy place, with wood-panelling on the walls and an eclectic mix of street signs and photographs from around Europe.