He's not bonkers. He spent a month pounding the pavements of the German capital ten hours a day for his new guidebook, Only In Berlin.

"Berlin feels exactly like Sheffield. The landscape is a mix of 19th century buildings, empty spaces, war damaged pockets and whopping great office blocks like that."

He gestures at the City Plaza opposite.

"Then there's the people. I was met by a friendly couple who were renting me a tiny little room in what was East Germany. In Vienna (he wrote a guide book on that city, too) the people are aloof."

Duncan, who uses his initials so when he's Googled people don't come up with a failed Tory leader, stirs his coffee.

"I felt a sense of liberation in the air, liberation from Communism. In Sheffield I still feel the buzz that we are losing that old grim Victorianism."

We can take it that former Sheffielder Duncan, aged 47, who now lives in Vienna but visits his parents in Brincliffe Edge every summer, has not written the usual kind of guidebook.

"I try to find the hidden places. There is no point in my writing another Lonely Planet or Rough Guide because the shelves are already groaning with them."

Duncan, who uses the Urban Explorer tag because that's what he does, had the problem of how to cope with Hitler and The Wall without going over old ground.

Luckily, he was just around the Corner from Checkpoint Charlie, until the fall of Communism the main way into East Berlin and the German Democratic Republic.

It's a tourist attraction but on his street there was just a line on the road.

Not far away was the site of an old Nazi building, now partly used as a children's playground and a Chinese takeaway. It was also the site of Hitler's bunker.

"That's how I dealt with the Nazis. Hitler would have been appalled.

All that talk about the Asiatic hordes and there's a Chinese takeaway..."

Eerily, one night he found the site of the bunker outlined in candles. He never solved the mystery.

Duncan, who has just seen the reissue of his book Yorkshire - A Portrait in Colour reprinted for the eighth time since 1995, unearthed little curiosities.

At a natural history museum the curator showed him the Madagascan Purple Swamp Hen, purchased in 1819 from a former Sheffield goldsmith, William Bullock.

He found there were five, not two, Brothers Grimm collecting fairy tales, poked his nose down sewers and discovered an abandoned pneumatic postal system.

He also put himself in hospital. Not for him schmoozing around a city, he does it on a shoestring.

"All I know is that by the time I finish I get very thin. On the last but one day I got a kidney stone, staggered into a chemist and they put me in hospital."

Berlin follows books on Vienna, Prague and Budapest. Munich is next.
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