‘Second, I myself have some appreciation of the world of the homosexual. The late Dr Magnus Hirschfeld wrote several books on the subject, one of which I’m proud to say I published myself. It’s a secret and rather treacherous world, Herr Gunther. A blackmailer’s charter. So it may be that this evil person is actually acquainted with my son. Even between men and women, love can make a good reason for blackmail — more so when there is adultery involved, or race defilement, which seems to be more a cause for concern to these Nazis.

‘Because of this, when you have discovered the blackmailer’s identity, I will tell Reinhard, and then it will be up to him what is to be done. But until then he will know nothing of this.’ She looked at me questioningly. ‘Do you agree?’

‘I can’t fault your reasoning, Frau Lange. You seem to have thought this thing through very clearly. May I see the letters from your son?’ Reaching for a folder by the chaise she nodded, and then hesitated.

‘Is that necessary? Reading his letters, I mean.’

‘Yes it is,’ I said firmly. ‘And do you still have the notes from the blackmailer?’ She handed me the folder.

‘Everything is in there,’ she said. ‘The letters and the anonymous notes.’

‘He didn’t ask for any of them back?’

‘No.’

‘That’s good. It means we’re dealing with an amateur. Someone who had done this sort of thing before would have told you to return his notes with each payment. To stop you accumulating any evidence against him.’

‘Yes, I see.’

I glanced at what I was optimistically calling evidence. The notes and envelopes were all typewritten on good quality stationery without any distinctive features, and posted at various districts throughout west Berlin — W.35, W.40, W.50 — the stamps all commemorating the fifth anniversary of the Nazis coming to power. That told me something. This anniversary had taken place on 30 January, so it didn’t look like Frau Lange’s blackmailer bought stamps very often.

Reinhard Lange’s letters were written on the heavier weight of paper that only people in love bother to buy — the kind that costs so much it just has to be taken seriously. The hand was neat and fastidious, even careful, which was more than could be said of the contents. An Ottoman bath-house attendant might not have found anything particularly objectionable about them, but in Nazi Germany, Reinhard Lange’s love-letters were certainly sufficient to earn their cheeky author a trip to a KZ wearing a whole chestful of pink triangles.

‘This Dr Lanz Kindermann,’ I said, reading the name on the lime-scented envelope. ‘What exactly do you know about him?’