The tube was one of the easiest places to get lost in London. The combination of tunnels, lifts, escalators, and seven lines built independent of any order made it a nightmare to navigate for the uninitiated. However, Knox knew that spies of every nationality spent long hours learning the tricks and shortcuts of the subterranean maze, and he wanted to take extra precautions before he reunited with Williams just in case someone was tailing him.

Harrods was its own kind of riddle. The vast emporium was a knot of departments that wanted to sell the fabulously wealthy anything their hearts desired as soon as they stepped through its doors. And its meandering pathways were designed to make it as hard as possible for them to leave again. Knox knew it almost as well as he knew the tube.

MI5 had quietly used Harrods as a training ground for decades, and both Knox and Williams had spent their fair share of time trying to slip through the book department or across the antiques gallery without being spotted before retiring to the food hall for a slice of meat pie or a sandwich.

Knox tested his muscle memory now, weaving between perfume counters, behind pillars and up and down gaudy escalators and staircases for almost half an hour. He even stopped briefly in the food hall to buy a large Melton Mowbray pork pie. Unfortunately for him, the three Soviet illegals who had tracked him to Knightsbridge were wise to his game and simply waited outside, watching from the far side of the Brompton Road until he emerged again and headed down into the tube once more.

The Russian agents shadowed Knox in rotation, each one only riding for a few stops in the same carriage as him before swapping places with each other. When Knox went above ground at King’s Cross two of them followed him up into the railway station, taking off their jackets and folding them over their arms for a quick and simple change of appearance, then followed him back down into the tube ticket hall, where their comrade was waiting for them, and onto the Piccadilly line to Holborn and the Central line back to Marble Arch.

They followed him, keeping their distance, to his Jaguar. Two of them got into the black Ford Cortina that they’d also driven from Mayfair an hour and a half ago, while the third peeled off and disappeared towards Hyde Park.