KATE WOKE EARLY and sat at the kitchen table, drinking tea and going over the provisional plan she had come up with the night before with the aid of an old London A–Z she’d located on Stuart’s bookshelf. It still seemed the most viable option in the cold dawn light.

She assumed that, if MI5 was tailing not only her but the head of the Service, they would have secured the legal permission to deploy a range of measures from the home secretary, including electronic interception – which was why she’d needed the A–Z, rather than Google Maps. But this gave her the chance to lull them into a false sense of security, too, particularly if they felt she was heading somewhere expected and routine.

So, she began the day with an early call to Dr Wiseman’s secretary, asking for an urgent appointment. She left a message, explaining she’d had another night without sleep and would be coming up to Ealing now, in the hope he could fit her in.

Then she left the house on foot, with a gym bag over her shoulder. The last impression she wanted to leave her watchers was of a woman off on a lengthy journey.

She walked north, skirting Battersea Park, crossing the river, then heading east with the flow of commuter traffic. She wove through Pimlico and finally disappeared underground and into the tube at Victoria. She’d estimated a team of at least four, but probably no more – resources were always tighter than anyone might imagine, even in the search for traitors – and she spotted the first man in the surveillance team as she swerved away from what seemed a full carriage at the last minute and ducked into the next one along. He was bald and wore blue jeans and a green Patagonia fleece, but he mistimed his lunge and only just made it through the doors. One or two commuters looked at him in surprise as he stared at the floor. Kate smiled. Amateur.

She picked out the second in the next carriage along once she had changed trains to the Central line at Oxford Circus. He was leaning against the side, reading a copy of The Times. He was wearing brown walking shoes, a grey outdoor shirt and a Gore-Tex jacket. But who goes to work in rush-hour dressed like that?

She’d noticed in the transfer between lines at Oxford Circus that the bald-headed man had disappeared from view. Clearly he’d sensed he’d been spotted. She moved closer to the man in the Gore-Tex jacket now, as if to let him know that she’d sussed him out, too. He paid no attention.