‘You were studying Czech?’

‘History. Most of the courses were in English, but I did learn Czech.’

They were in front of an impressive Rococo building now. ‘This is the Kinsky Palace,’ he said, ‘which was where the Communist leader Klement Gottwald effectively launched the Communist coup d’état in 1948. Do you actually want to know any of this stuff?’

‘Just keep on talking.’

They emerged into the Old Town Square just as the sun slipped beneath the Gothic spires of the Church of Our Lady Before Tyn.

The square was crowded with tourists waiting for the famous astronomical clock to strike the hour. A piano player entertained some adults in one corner, while families with children were clustered around a man dressed in a giant panda suit and a clown blowing bubbles into the still evening sky. ‘I love the south side of this square,’ Callum said, with something approaching rapture. ‘Most of these houses have Romanesque or Gothic origins, but they’re such a jumble of beautiful overlapping styles.’ He took her closer. ‘This is my favourite, the Storch House.’ Kate looked up at the beautiful paintings that adorned its façade.

Callum glanced at his watch. ‘You should probably see the clock strike.’

They walked over to the other side of the square and waited with the tourists in front of the town hall. ‘All right,’ Kate said. ‘Apart from the obvious, why is it called the astronomical clock?’

‘Because as well as the time, it displays the movement of the sun and moon through the signs of the zodiac, and of the planets around the Earth.’ They watched as the beautiful, elaborate and ornate clock began to strike. ‘Now you see the procession of the twelve Apostles,’ Callum said. ‘I must have witnessed this a thousand times, but I still love it.’

Kate had lost interest in the clock again, because she was nearly certain now that the man loitering by the entrance to Melantrichova Street back on the south side of the square was the short, slight, dark-haired man of Middle Eastern appearance she had seen passing them in the passenger seat of the Prius at the Powder Gate. It was notoriously difficult for surveillance teams to pass themselves off as tourists without fair warning, especially of the kind that milled slowly around central Prague. And he was failing in the task. ‘Okay, rapture over,’ Kate said. ‘Let’s move on.’

‘Where do you want to go?’

‘Just keep the tour ticking over. I’m making some progress. And let’s speed up a bit.’

As they moved down the tight, winding Charles Street, with its pretty houses, Baroque façades and stucco reliefs, speed was easier said than done. The tourist throng was thicker here and slower moving, all the way on to the Charles Bridge, with its magnificent view of the castle above. ‘Let’s wait,’ Kate said. ‘Tell me about it.’

‘I thought we were in a hurry?’

‘Do as you’re told, Callum.’

He gave a barely perceptible sigh. ‘What do you want to know?’