Somewhere along Walworth Road, between the Elephant and Castle and the Richardson gang's old scrap-metal yard, wiped from physical existence now by a council estate, a feeling of guilt sets in.
Taxi drivers beep their car horns at our driver as our minibus passes by. Market traders wave a cheery hello before turning back to their stalls. And we laugh, nervously, as our guide, "Mad" Frankie Fraser, tells us he used to be the best "dentist" in London.
Above the chatter of the other passengers, snippets of a gangland life punctuated with beatings, slashings, shootings, prison officers and guv'nors float across their heads to reach me at the back of the minibus. "That's the old Carter Street police station where I got a few whacks of the truncheon," says Frankie.
"All your best thieves come from south London" and "guns, knives, bottles . . . bosh, bosh . . . I was determined to kill him".
As the minibus turns into the Elmington estate, where the Great Train Robbery was planned our driver, Marilyn Wisbey, daughter of the train robber, Tommy Wisbey, and now Frankie's girlfriend, lets out a throaty laugh. Frankie is telling another gangland story. It is about when Marilyn's mother, a wonderful seamstress, he says, ran up the balaclavas the gang wore when they held up the train.
"She didn't know what they were for, mind you, but it wasn't for a fancy dress ball," laughs Frankie, and the others laugh back. Except, that is, for the young woman in front of me who looks horrified with herself for just being here. It seems her earlier eagerness "for a bit of housewives' titillation - you know, hanging out with gangsters" has given way to severe embarrassment.
Of course Frankie is talking about the good old days. When women and children could walk the streets of London without fear. When Frankie's friends Ronnie and Reggie Kray ruled the East End. And when Frankie was the "enforcer" for the Richardson gang and had a disturbing fondness for pliers, with which he would pull out the teeth of fellow hoodlums if they crossed his "firm".
Now Ronnie has gone to meet his Maker. Reggie's friends are still trying to get him out of prison and Frankie and Marilyn are charging £30 a head for a Gangland Tour around London's crime "theme park".
"I don't have any regrets," says Frankie of the men he killed ("take a number out of a hat") as we stand beside the ring at Repton Boys' Boxing Club where he boxed with the Krays when they were kids in Bethnal Green. The sun shines into the gym through a skylight and illuminates the left side of Frankie's 76-year-old face and I watch his eyes.
They're not the bottomless, black eyes of a shark of earlier in the day. They seem translucent, strong and clear. He stares right back at me and I ask him about having the power to end a life. "That was the road I went down and that was the road they went down," he says. "The only regret I have is that I was caught and that I didn't rob a bigger bank."
Frankie is a funny, charming little man, who justifies his crimes as some kind of twisted honour among thieves. Dressed up in his dark suit and mustard-coloured waistcoat, his knockabout, London Palladium-style humour belies an unimaginable thought process. The logic of Frankie's gangland world is that so long as no one else gets hurt, the "hard men" can just go right ahead and shoot, murder, slash, axe and smash each other to pieces.
Some of the people on the minibus tour - made up of middle-class couples and a few young lads - express a certain understanding of Frankie's logic. "They didn't go around shooting anyone, they kept it among themselves," says one woman. Others, who readily admit joining the tour out of a morbid sense of fascination, are beginning to feel a little uncomfortable.
As we stop for a drink at the Grave Maurice pub, which is arguably the roughest in London (a poster on the wall invites people to buy tickets for a "booze and meat" raffle) one man in the group says he has been trapped in a bizarre cartoon. Frankie and Marilyn have brought us to the Grave Maurice because this is where he used to meet the Krays to discuss tactics, but my fellow passenger is experiencing a mixture of guilt and fascination.
"This is so weird", says the man. "I feel guilty laughing, but I can't help it. It's all so strange."
Outside in the Whitechapel Road it is the end of the tour. Frankie poses for a few photographs and there is still a look of "Am I really here?" on the faces of some of the gangland tourists. Marilyn laughs a bit more at Frankie's stories, then he asks us to write to the Home Secretary to try to get Reggie out of prison.