Part of the reason why using Dr Wiseman had appealed to her was that Ealing Broadway lay at the end of the line. And, since she had also picked the very last carriage in the change at Oxford Circus, the numbers began to thin out beyond White City. The man in the Gore-Tex jacket got off at West Acton. She saw him muttering into his lapel mic as he walked away down the platform.

Kate sat down to survey the rest of the passengers. There was an art, of course, in doing so without being seen to do it, which made it a slow process.

As the train rattled towards its final destination, she had mentally given the remaining passengers the all-clear, but she changed her mind at the last moment about the couple at the far end of the carriage. They’d been chatting amicably, a passable imitation of lovers going to work, but what were the chances of two people in their twenties heading to work at the same tube station at the same time?

A workplace romance, perhaps. But somehow she doubted it. Their conversation had been too intense, too consistent. Couples who were that garrulous were also physically all over each other, in the first flush of love. She made a mental note to check that the pair did not appear anywhere on the journey beyond Dr Wiseman’s office.

She left the train at Ealing Broadway and walked down the platform towards the barriers. She kept her gait easy and relaxed – it had long been a part of surveillance training to learn instinctively to spot changes in how a suspect held him- or herself – and, ten minutes later, hurried through Dr Wiseman’s reception without so much as a sideways glance.

The doctor’s secretary was on the phone, so, without greeting or explanation, Kate walked on to the bathroom at the back. She closed the door, opened the window, climbed through and dropped noiselessly into the suburban garden beyond. It was tougher to get over the fence on the other side without incident – she ended up having to commandeer a wheelbarrow – but she calculated she was into the street beyond in less than thirty seconds.

She allowed herself to look back, then carried on briskly along the route she’d memorized from the A–Z. Her phone, now switched off – as you would expect of a woman in an appointment with her psychiatrist – was no longer able to give away her progress.

The only part of the operation she had not been able to plan was how exactly she would get from there to Heathrow, but in the end her luck was in and she hailed a taxi shortly after she re-joined Madeley Road in search of a bus.