He must have had superhuman hearing, for it was only after we'd stood there for a moment or two that I could hear the sounds of voices and a muffled hammering. Frank put his head close to mine and whispered, 'Sounds travel a long way in these old tunnels. Those men are probably no nearer than the old disused platform at Französischestrasse.' He looked round. 'This is where you leave me.' He pointed up to another air shaft. At the top there was the faintest glimmer of grey light seen through a grating. 'But move quietly.'

I stripped off the overalls and passed them to Frank; then I climbed up the narrow air shaft. There were iron rungs set into the brickwork. Some of them were rusted and broken, but I had nothing to carry and I got to the top easily enough. The grating was held in place with rusting bars. It looked immovable.

'Lift it,' said Frank from below me. 'Lift it until you can see the street is clear. Then choose your moment and go.'

I put my hand to the grating and it moved easily enough. It hadn't been cleaned and oiled – Frank was too subtle for anything so obvious – but it had been removed recently and made ready for me to push aside.

'Good luck, Bernard.'

I tossed my working gloves back down the shaft, and then went through the manhole as quickly as I could, but I need not have worried. The Friedrichstadt – the governmental centre of old Berlin – is empty and silent by Western standards, even during the working day. Now there was no one in sight, just the distant sounds of traffic somewhere to the east of the city. For Stadtbezirk Mitte is a Communist fist punched into the West. It is bordered on three sides by the 'anti-Fascist protection barrier', or what the rest of the world calls the Wall. It was close by. Endless batteries of glaring lights kept the open strip of borderland as bright as day, and the scattered light made the darkness overhead grey, like the mist that creeps inland from an ice-cold sea.

Frank had chosen my route with care. The entrance to the air shaft was hidden from passers-by. There was a pile of sand and big heaps of rubble, some building equipment and a small generator trailer belonging to the Electricity Authority. Berlin 's cast-iron manhole covers are very heavy, and by the time this one was back in place I was red-faced and out of breath. I paused for a moment before walking up Charlottenstrasse, intending to cut along the back of the State Opera House parallel to Unter den Linden. I would have to cross the Spree. There was no way of avoiding those bridges, for just as the Wall enclosed this part of Mitte on two sides, the River Spree made up the other two sides of what was virtually a box.

As I got nearer to the State Opera, I saw lights and people. Doors at the back of the building were open and men were carrying huge scenery flats and the statue of a horseman that was recognizably from the last act of Don Giovanni. I crossed the street to keep in the shadows but two policemen walking towards me from the direction of the old Reichsbank building – now the offices of the Central Committee – made me change my mind quickly. If only we hadn't had to wait until the underground trains stopped, I could have mingled with the tourists and those groups of Western visitors who go through Checkpoint Charlie just to visit the theatres or the opera houses for the evening. Some of them were dressed in dinner suits and stiff-fronted shirts, or the flamboyant mess kit of a garrisoned regiment. With them came women in long evening dresses and expensive hairdos. Such visitors provided a glimpse of Western decadence for the bored locals. None of those visitors ever gets asked for papers on the street, but such dress would be rather conspicuous amongst the workers where I was going.

There were very few people to be seen anywhere. I walked north and stopped under the arch at Friedrichstrasse station. There were a couple of noisy men arguing about the satirical cabaret across the road, some railway workers waiting for their shift to begin and some silent African tourists staring at everything. The Weidendamm bridge would be my best bet. It was darker there than on the bridges that went over to the island; too many government buildings being guarded on that side of the city.

There were memories everywhere I looked, and there was no getting away from the war. The last escapers from the Füihrerbunker had come this way, crossing the river by the footbridge when all else failed, and leaving Martin Bormann dead by the river.

The Charité Hospital. In the mortuary of that grim building, the Red Army found the bodies of the men who had tried to overthrow Hitler in the July 1944 plot. Their bodies had been kept in the cold room there on Hitler's personal orders.

A policeman came walking up from the old Brecht theatre beside the Spree. He hurried his pace as he saw me. My papers were in order but I realized too late that I didn't know how to talk to a policeman. 'Hey, you,' the policeman called.

How did East Berliners address a policeman nowadays? This wasn't the USA. Being too familiar would be just as suspicious as being too respectful. I decided to be a little drunk, a shift-worker who'd had a couple of vodkas before heading home. But how many vodkas could a man have these days before he risked being taken to the police station?

'What are you doing here?' The policeman's voice was shrill, and his accent revealed his home to be somewhere in the north: Rostock, Stralsund or Rügen Island, perhaps. On this side of the Wall there was a theory that out-of-town recruits were more reliable than Berliners.

I kept walking. 'Get up,' the policeman said. I stopped and turned round. He was talking to a couple of men sitting on the ground in the shadow of the bridge. They didn't get up. The cop said, 'Where are you from?'

The elder of the two, a bearded man wearing overalls and a battered leather jacket, said, 'And where are you from, sonny?'

'Let's get you home,' said the cop.

'Get me home,' said the bearded man. 'That's right. You get me home to Schöneberg.' He laughed. 'Yorckstrasse, please, right near the railway.'

The younger man got to his feet unsteadily, 'Come on,' he said to his companion.

'Yorckstrasse, Schöneberg,' said the bearded man again. 'Only two stops from here on the S-Bahn. But you've never heard of it and I'll never see it again.' He began to sing tunelessly. 'Das war in Schöneberg im Monat Mai.' His singing voice revealed the extent of his drunkenness in a way that his speech did not.

The policeman was less conciliatory now. 'You'll have to get off the street,' he said. 'Stand up. Show me your papers.'

The drunk gave an artful little laugh. His companion said, 'Leave him alone – can't you see he's not well,' in a voice so slurred that his words were almost incomprehensible.

'If you're not on your way home in two minutes, I'll run you along to the police station.'

'Er ist polizeiwidrig dumm,' said the bearded man, and laughed. It meant criminally stupid, and it was a joke that every German policeman had heard.

'Come along with me,' said the cop.

The man began singing again, louder this time: 'Das war in Schöneberg im Monat Mai…'

I hurried on lest the policeman call for help with his two difficult drunks. Even when I was a hundred metres or more down the road, I could still hear the drunken old man singing about the little girl who had so often and gladly kissed the boys as they did in Schöneberg so long ago.

At Oranienburger Tor, where the Chausseestrasse leads up to the football stadium, I turned into the dark labryinth of side streets. I'd forgotten what it was like to be a newly 'deposited' field agent with false papers and a not very convincing cover story. I was too old for it; once I was safely back behind my desk in London, I wouldn't fret to move again.