They were near the Potsdamer Platz when they heard a signal. Macke recognized the characteristic sound. ‘That’s a pianist,’ he said with relief. At least he could prove to Werner that the equipment worked. Someone was broadcasting five-digit numbers, one after the other. ‘Soviet Intelligence uses a code in which pairs of numbers stand for letters,’ Macke explained to Werner. ‘So, for example, 11 might stand for A. Transmitting them in groups of five is just a convention.’

The radio operator, an electrical engineer named Mann, read off a set of co-ordinates, and Wagner drew a line on a map with a pencil and rule. Richter put the van in gear and set off again.

The pianist continued to broadcast, his beeps sounding loud in the van. Macke hated the man, whoever he was. ‘Bastard Communist swine,’ he said. ‘One day he’ll be in our basement, begging me to let him die so the pain will come to an end.’

Werner looked pale. He was not used to police work, Macke thought.

After a moment the young man pulled himself together. ‘The way you describe the Soviet code, it sounds as if it might not be too difficult to break,’ he said thoughtfully.

‘Correct!’ Macke was pleased that Werner caught on so fast. ‘But I was simplifying. They have refinements. After encoding the message as a series of numbers, the pianist then writes a key word underneath it repeatedly – it might be Kurfürstendamm, say – and encodes that. Then he subtracts the second numbers from the first and broadcasts the result.’

‘Almost impossible to decipher if you don’t know the key word!’

‘Exactly.’

They stopped again near the burned-out Reichstag building and drew another line on the map. The two met in Friedrichshain, to the east of the city centre.

Macke told the driver to swing north-east, taking them nearer to the likely spot while giving them a third line from a different angle. ‘Experience shows that it’s best to take three bearings,’ Macke told Werner. ‘The equipment is only approximate, and the extra measurement reduces error.’

‘Do you always catch him?’ said Werner.

‘By no means. In most cases we don’t. Often we’re just not quick enough. He may change frequency halfway through, so that we lose him. Sometimes he breaks off in mid-transmission and resumes at another location. He may have lookouts who see us coming and warn him to flee.’

‘A lot of snags.’

‘But we catch them, sooner or later.’

Richter stopped the van and Mann took the third bearing. The three pencil lines on Wagner’s map met to form a small triangle near the East Station. The pianist was somewhere between the railway line and the canal.

Macke gave Richter the location and added: ‘Quick as you can.’

Werner was perspiring, Macke noticed. Perhaps it was rather hot in the van. And the young lieutenant was not accustomed to action. He was learning what life was like in the Gestapo. All the better, Macke thought.