The interior wasn’t quite what I had expected of Sipo HQ. We waited in a hall, the central feature of which was an ornate gilt staircase, decorated with fully-formed caryatids, and enormous chandeliers. I looked at Nebe, allowing my eyebrows to inform him that I was favourably impressed.

‘It’s not bad, is it?’ he said, and taking me by the arm he led me to the French windows which looked out on to a magnificent landscaped garden. Beyond this, to the west, could be seen the modern outline of Gropius’s Europa Haus, while to the north, the southern wing of Gestapo headquarters on Prinz Albrecht Strasse was clearly visible. I had good reason to recognize it, having once been detained there awhile at Heydrich’s order.

At the same time, appreciating the difference between the SD, or Sipo as the Security Service was sometimes called, and the Gestapo was a rather more elusive matter, even for some of the people who worked for these two organizations. As far as I could understand the distinction, it was just like Bockwurst and Frankfurter: they have their special names, but they look and taste exactly the same.

What was easy to perceive was that with this building, the Prinz Albrecht Palais, Heydrich had done very well for himself. Perhaps even better than his putative master, Himmler, who now occupied the building next door to Gestapo headquarters, in what was formerly the Hotel Prinz Albrecht Strasse. There was no doubt that the old hotel, now called SS-Haus, was bigger than the Palais. But as with sausage, taste is seldom a question of size.

I heard Arthur Nebe’s heels click, and looking round I saw that the Reich’s crown prince of terror had joined us at the window.

Tall, skeletally thin, his long, pale face lacking expression, like some plaster of Paris death-mask, and his Jack Frost fingers clasped behind his ramrod-straight back, Heydrich stared outside for a moment or two, saying nothing to either of us.

‘Come, gentlemen,’ he said eventually, ‘it’s a beautiful day. Let’s walk a bit.’ Opening the windows he led the way into the garden, and I noticed how large were his feet and how bandy his legs, as if he had been riding a lot: if the silver Horseman’s Badge on his tunic pocket was anything to go by, he probably had.

In the fresh air and sunshine he seemed to become more animated, like some kind of reptile.