At last they came out into a small courtyard and crossed over to some crumbling mews buildings. In the days when transport had still been pulled by horses these had been stables, but they had long since been converted into garages for motor vehicles.

They immediately saw that Mark had already been here. Been and gone. The doors to the garage were hanging open, the padlock lying broken on the cobbles.

The garage itself was empty.

‘We’re too late.’ Perry shook his head and looked at James. ‘If he has a smash, that’s the end of your m-motor.’

‘Never mind that,’ said James. ‘If he has a smash, that’s the end of him.’

‘But what can we do?’ said Perry, peering into the empty garage.

‘He can’t be long gone,’ said James, sniffing the air. ‘I can still smell fumes. He must have left only a few minutes ago. He won’t risk going through the centre of Windsor and back through Eton; it’s too crowded and he’d be stopped straight away if a beak saw him. The quickest way out of town from here is Albert Road towards Staines.’

‘Then there’s still a chance,’ said Perry. ‘It’ll be slow going. You saw how busy the roads are.’

‘Think we can catch him?’ asked James.

‘We can try,’ said Perry. ‘I know a short cut. Come on.’

Perry led James out of the courtyard and up a narrow flight of steps that led to a dark, twisting alley behind a row of shops. Halfway along Perry ducked into the back garden of a pub and then vaulted over a wall on the other side. James followed and found himself in a row of allotments. Perry steamed ahead past neat plots of beans and cabbages and out on to a long side street lined with lime trees. They pounded down the pavement and reached the main road just in time to see a car approaching and slowing for the junction.

The black-and-white Bamford and Martin was unmistakable: it was a sleek, open-topped roadster, built for speed. And the boy behind the wheel was unmistakable too, despite his tear-stained face and windswept hair.

‘There he is,’ James yelled and put on a burst of speed.

‘Be careful,’ Perry shouted after him as James sprinted across the road and threw himself into the path of the oncoming car, which came to a shuddering halt.

Mark sounded the horn and cursed. There was a wild fury in his eyes. His hands were gripping the steering wheel so tightly his knuckles were white.

‘Get out of the way, James,’ he said, his face screwed into an ugly scowl.

‘No,’ said James. ‘You’ve got to go back. This is crazy.’

‘Get out of my way,’ Mark repeated. ‘Or I’ll run you down.’

‘You wouldn’t do that,’ said James, but even as he said it, he was not so sure.

‘Move,’ Mark yelled, and he pressed his foot down on the accelerator, gunning the engine.

‘I’m not moving,’ said James, stepping backwards warily. ‘Don’t be stupid, Mark. Let’s take the car back.’