If Johnson is annoyed by this, he doesn't show it, and launches into his well-rehearsed Dick Barton patter. How he was delighted at the thought of a regular income - even if it was only pounds 18 a week. How they all used to laugh out loud at the absurdity of some of the scripts. Yet there is no Dick Barton memorabilia on view in his flat. There are photos of him in various stage productions and umpteen biogs of the theatre's great and good, but if Barton's in there, he's in deep cover. This is partly modesty - at 82 years old, Johnson's got better things to do than create a shrine to himself - but it may also reflect a lingering ambivalence about Dick Barton.
These days, actors are happy to become identified as a fictional character in a long-running soap, as it guarantees them a life of C-list celebrity stardom. But Johnson was the first person to whom this happened. He became famous simply because of the character he played. To the public he was Dick Barton. This made him uncomfortable, primarily because his acting talents went unrecognised, but also because of the prevailing snobbishness within the profession.
"I was aware that Val Gielgud [BBC's head of drama and brother of the more famous John] always hated the show for being so downmarket."
Johnson had been a well-established actor for more than 10 years before he was offered the part of Barton by the producer Martyn Webster.
"Once I had made sure I could still do other work, I eagerly accepted," he says. "Even though the money was poor, it was still regular work."
First broadcast on 7 October 1946 within months, millions of listeners were tuning in. Dick Barton became a phenomenon. It was anticipated that the show would attract an exclusively adult audience, but it rapidly became essential listening for a generation of children. Good news for the ratings was bad news for Johnson.
"Barton was a proper character at first," he says. "He drank, he smoked and had a girlfriend. As soon as the producers cottoned on to the fact we had a youth audience, they felt they had to become moral guardians."
Out went all the interesting vices. Despite feeling Barton had been emasculated, Johnson was never less than professional, week-in, week-out for two and a half years, he did just that. But if the public was lapping it up, Johnson was less than enthusiastic. His voice had become so familiar that he had become type-cast and many producers wouldn't touch him; the well-paid commercial spinoffs were artistically unsatisfying; worst of all, he felt that the BBC didn't want to dilute its best asset and was preventing him from getting other work without offering any compensation. So Johnson did the unthinkable. He left.
"I went straight into a West End play," he said. "Needless to say it flopped."
The BBC, meanwhile, tried to make him change his mind. "They asked me to name my price. I said: `pounds 100 per week.' They said: `It sounds like you want danger money.' `That's precisely what I want,' I replied. And that was the end of it."
Johnson went on to make many theatre and TV appearances, most recently in A Dance to The Music of Time and A Touch of Frost.
But nothing he has done has brought him anything approaching the recognition of Dick Barton. So does Britain's first soap star, who made next to nothing out of it, feel when he sees EastEnders actors making a small fortune? "I've never watched EastEnders," he replies. Noel Johnson always was one of the old school.
A stage version of `Dick Barton, Special Agent' is on at the Warehouse Theatre, Croydon (0181-680 4060)