Clarke laughed at the thought. ‘I went on an intelligence corps training course back when I was in the navy, but it’s certainly not my day job. I first met Henderson about four years ago. As well as standard radios like we have in the apartment, Imperial Wireless makes specialist equipment like radios for military aircraft and ships, directional beacons, scrambler systems and such like.

‘I’ve been selling that equipment into France for seventeen years and over that time I’ve got to know everyone in the business – from the officers in the French naval procurement office, all the way down to the admin staff in our rivals’ sales offices.

‘Every few months Henderson and I meet for a drink. He’s always interested in the latest technical developments from our rivals and—’

Clarke had to stop speaking because he’d turned into a road that was blocked off by wooden barriers. They were less than a hundred metres away from a major railway station and the courtyard in front of it was jammed with people. But the steps leading inside were cordoned off and soldiers stood at wrought-iron gates with their guns poised.

A flat-capped policeman strode importantly towards the car as Mr Clarke wound down his window.

‘Do you have reservations for a train?’ he asked curtly.

Clarke shook his head. ‘I’m just trying to get across town.’

The officer shrugged before dramatically pointing him back the way he’d come. ‘No access. Leave at once!’

‘Blast,’ Clarke said, as the front tyre scraped the kerb. ‘Rosie, pull the map from under the seat and work out another way out of the city.’

After a few hundred metres Mr Clarke pulled up outside a laundry and leaned across to study the map his daughter had unfurled in her lap. He’d travelled all over France, persuading department stores to stock Imperial Wireless radios, so he knew the roads well and it took less than half a minute to familiarise himself with an alternative route.

Once he was back on roads he knew, Mr Clarke continued his story.

‘Late last year I got wind of a new transceiver – that’s a radio set that transmits and receives – designed and built by a Frenchman called Luc Mannstein. He runs a family company that has made very expensive and very beautiful watches for more than two hundred years. But Mannstein is also a keen radio amateur. Like a lot of hobbyists he’d built his own radio equipment and aerials that could transmit all over the world; but things only got interesting when he decided to apply his skills as a watchmaker to the production of a miniature set.