'Go and breathe all over him,' I said.

Werner smiled. Werner had proved what a brilliant field agent he could be. Werner was very very happy.

He made a fuss of course. He wanted his lawyer and wanted to talk to his boss and to some friend of his in the government. I knew the type only too well; he was treating us as if we'd been caught stealing secrets for the Russians. He was still protesting when he departed with the arrest team. They were not impressed; they'd seen it all before. They were experienced men, brought in from the BfV's 'political office' in Bonn.

They took him to the BfV office in Spandau, but I decided they'd get nothing but indignation out of him that night. Tomorrow perhaps he'd simmer down a little and get nervous enough to say something worth hearing before the time came when they'd have to charge him or release him. Luckily it was a decision I wouldn't have to make. Meanwhile, I decided to go and see if there was anything to be got out of the woman.

Werner drove. He didn't speak much on the journey back to Kreuzberg. I stared out of the window. Berlin is a sort of history book of twentieth-century violence, and every street corner brought a recollection of something I'd heard, seen, or read. We followed the road alongside the Landwehr Canal, which twists and turns through the heart of the city. Its oily water holds many dark secrets. Back in 1919, when the Spartakists attempted to seize the city by an armed uprising, two officers of the Horse Guards took the badly beaten Rosa Luxemburg – a Communist leader – from their headquarters at the Eden Hotel, next to the Zoo, shot her dead and threw her into the canal. The officers pretended that she'd been carried off by angry rioters, but four months later her bloated corpse floated up and got jammed into a lock gate. Now, in East Berlin, they name streets after her.

But not all the ghosts go into this canal. In February 1920 a police sergeant pulled a young woman out of the canal at the Bendler Bridge. Taken to the Elisabeth Hospital in Lützowstrasse, she was later identified as the Grand Duchess Anastasia, the youngest daughter of the last Czar of All the Russias and only survivor of the massacre.

This is it,' said Werner, pulling into the kerb. 'Good job there's a cop on the door, or we'd come back to find the car stripped to the chassis.'

The address the contact had given was a shabby nineteenth-century tenement in a neighbourhood virtually taken over by Turkish immigrants. The once imposing grey stone entrance, still pitted with splinter damage from the war, was defaced by brightly coloured graffiti sprays. Inside the gloomy hallway there was a smell of spicy food and dirt and disinfectant.

These old houses have no numbered apartments, but we found the BfV men at the very top. There were two security locks on the door, but not much sign of anything inside to protect. Two men were still searching the hallway when we arrived. They were tapping the walls, prizing up floorboards, and poking screwdrivers deep into the plaster with that sort of inscrutable delight that comes to men blessed by governmental authority to be destructive.

It was typical of the overnight places the KGB provided for the faithful. Top floors: cold, cramped and cheap. Perhaps they chose these sleazy accommodations to remind all concerned about the plight of the poor in the capitalist economy. Or perhaps in this sort of district there were fewer questions asked about comings and goings by all kinds of people at all kinds of hours.

No TV, no radio, no soft seats. Iron bedstead with an old grey blanket, four wooden chairs, a small plastic-topped table and upon it black bread roughly sliced, electric ring, dented kettle, tinned milk, dried coffee, and some sugar cubes wrapped to show they were from a Hilton hotel. There were three dog-eared German paperback books – Dickens, Schiller, and a collection of crossword puzzles, mostly completed. On one of the two single beds a small case was opened and its contents displayed. It was obviously the woman's baggage: a cheap black dress, nylon underwear, low-heeled leather shoes, an apple and orange, and an English newspaper – the Socialist Worker.

A young BfV officer was waiting for me there. We exchanged greetings and he told me the woman had been given no more than a brief preliminary questioning. She'd offered to make a statement at first and then said she wouldn't, the officer said. He'd sent a man to get a typewriter so it could be taken down if she changed her mind again. He handed me some Westmarks, a driving licence, and a passport; the contents of her handbag. The licence and passport were British.

'I've got a pocket recorder,' I told him without lowering my voice.

'We'll sort out what to type and have it signed after I've spoken with her. I'll want you to witness her signature.'

The woman was seated in the tiny kitchen. There were dirty cups on the table and some hairpins that I guessed had come from a search of the handbag she now held on her lap.

'The captain tells me that you want to make a statement,' I said in English.

'Are you English?' she said. She looked at me and then at Werner. She showed no great surprise that we were both in dinner suits complete with fancy cuff links and patent-leather shoes. She must have realized we'd been on duty inside the house.

'Yes,' I said. I signalled with my hand to tell Werner to leave the room.

'Are you in charge?' she asked. She had the exaggerated upper-class accent that shop girls use in Knightsbridge boutiques. 'I want to know what I'm charged with. I warn you I know my rights. Am I under arrest?'

From the side table I picked up the bread knife and waved it at her. 'Under Law 43 of the Allied Military Government legislation, still in force in this city, possession of this bread knife is an offence for which the death sentence can be imposed.'

'You must be mad,' she said. 'The war was almost forty years ago.'

I put the knife into a drawer and slammed it shut. She was startled by the sound. I moved a kitchen chair and sat on it so that I was facing her at a distance of only a yard or so. 'You're not in Germany,' I told her. 'This is Berlin. And Decree 511, ratified in 1951, includes a clause that makes information gathering an offence for which you can get ten years in prison. Not spying, not intelligence work, just collecting information is an offence.'

I put her passport on the table and turned the pages as if reading her name and occupation for the first time. 'So don't talk to me about knowing your rights; you've got no rights.'

From the passport I read aloud: 'Carol Elvira Miller, born in London 1930, occupation: schoolteacher.' Then I looked up at her. She returned my gaze with the calm, flat stare that the camera had recorded for her passport. Her hair was straight and short in pageboy style. She had clear blue eyes and a pointed nose, and the pert expression came naturally to her. She'd been pretty once, but now she was thin and drawn and – in dark conservative clothes and with no trace of makeup – well on the way to looking like a frail old woman. 'Elvira. That's a German name, isn't it?'

She showed no sign of fear. She brightened as women so often do at personal talk. 'It's Spanish. Mozart used it in Don Giovanni.'

I nodded. 'And Miller?'

She smiled nervously. She was not frightened, but it was the smile of someone who wanted to seem cooperative. My hectoring little speech had done the trick. 'My father is German… was German. From Leipzig. He emigrated to England long before Hitler's time. My mother is English… from Newcastle,' she added after a long pause.

'Married?'

'My husband died nearly ten years ago. His name was Johnson, but I went back to using my family name.'

'Children?'