Yet Streets Of London is but one detail of a much wider picture of Ralph May, who, as a Croydon teenager, adopted his stage surname as a salaam to a Mississippi blues legend. He then took to the road like dustbowl balladeer Woody Guthrie, ‘armed only with a guitar and a pocketful of dreams', just before the hunt was up in the mid-1960s for a British answer to Bob Dylan.
Ralph didn't get the job, but he came within an ace of a No 1 when Streets Of London, a six-year-old album track, was issued in time for 1974's Christmas sell-in. By then, however, a hit record was a mere sideshow to McTell, who is still filling European theatres, and commands a vast fan base for whom the release of each new album remains a special event.
G&B: What matters when you're assessing a guitar?
RMcT: I have to have a good bass resonance, just as a choir, brass band or rock group would. It's easy to get satisfactory treble, but not so easy to find the warmth and depth of the lower strings, particularly as I play fingerstyle, and every area of the instrument is doing what it can to assist the vocal. I'm wedded to an old Gibson J-45, one of the finest for accompanying vocals, which I bought in 1966. I'm going to have a new fretboard put on - its third - and that one'll be a little wider than at present. It's been messed about so much that it has no intrinsic value, but it's priceless to me.
Is there any differentiation between what guitars you use in the studio and on the stage?
It really doesn't matter, although I've got a 1934 00-sized Gibson which has a louder bass end when you record than the J-45, which is twice the size - so much so that it'll distort if you put it too near a microphone. In the studio, I tend to play more without a thumbpick - which I've used all my life to state the bottom end of the guitar with more clarity - but you tend to get a better recorded sound with just using your fingernails.
Were you ever tempted to go electric?
Not until I heard Ry Cooder - who invented something timeless through a synthesis of all sorts of styles that aren't traceable back to a specific person. So I bought a Strat and a chorus pedal. I also admire Mark Knopfler because, like Ry, he obviously adores the acoustic guitar.
Growing up in Croydon, did you get sucked into the blues movement that spawned groups like the Stones, the Yardbirds and the Pretty Things?
I used to enjoy The Yardbirds at the Star and John Mayall's Bluesbreakers at Eel Pie Island as well as Cyril Davies and Long John Baldry. I also saw urban blues revival packages with Muddy Waters and Big Joe Williams at Croydon's Fairfield Hall. Around that time some musicians chose the electric Chicago route, but there was a network of us who stuck to acoustic guitar and swapped scratchy old discs from the 1920s and 1930s.
How did you learn to play?
Essentially, it was from trying to copy records and refusing to be beaten. What was wonderful about the British acoustic guitar school was that it evolved a character of its own - no-one knew precisely what the old American blues and folk artists on the records were doing. You couldn't see them, so you had to work out your own chords. I thought I had Blind Blake's West Coast Blues nailed when I was 19, but three years ago, I was delighted to finally stumble upon what he was actually doing. Another example: I was taught Hesitation Blues by Gary Peterson, a Californian who'd learned it himself at the knee of its originator, the Reverend Gary Davis, but much later I heard Davis himself do it, and it was nothing like the way I played it!
Davy Graham, Bert Jansch, Martin Carthy and Wizz Jones were in the same boat. Each of us found his own ways of getting the sounds - although I had a mate who, quite unashamedly, would watch from the audience and draw performers' chord positionings in a notebook.
Are there any guitarists from that period that were undervalued?
Certainly, Gary Peterson was. He played fabulous ragtime guitar. We met in Paris, and he came over to live in Croydon - where he later formed Formerly Fat Harry. He made me realise that it was possible to play independently with your fingers without repeating patterns.
As well as blues and folk, there are other genres in your music. For instance, there's a pronounced strain of Gallic chanson...
That's a great compliment. I don't know much about that, though I lived in France for a bit, and I was introduced to Georges Brassens by the late Jake Thackray, a founding father of English chanson. I'm very broad in my musical tastes - very emotional too. All kinds of music can overwhelm me to the point of tears.
While accompaniment is totally integral to my own writing, what's the point of creating a song if it doesn't say something? My friend Eric Bogle reckons that music's aimed at the heart, the head, the feet or the groin. I like to make music that makes people think or react emotionally - though I don't mind if it hits them in the groin, too.
Overall, I prefer melody to interesting chord sequences - though it's often just a single chord that makes a whole song important. There's a dearth of good melody writers. That's a shame because melody gives you facilities to improvise, to write counter-melodies, to harmonise.
Do you ever feel frustrated by your own stylistic cliches?
No, because I can see them coming a mile off. In my early days I nearly always wrote the tune first, then kept playing it over and over again until a lyrical idea came to me. Often, what make sense musically isn't always the best vehicle for words, but it's a challenge I enjoy.
I've been trying not to mention Streets Of London...
[Laughs] Well, I would have done it for you. It's like my comfort blanket. That was a ‘young' song that elicited instant response - but one of the most satisfying songs I've ever written has been The Setting, presented like a short story and in an open tuning. Yet it took almost a decade after it appeared on Bridge Of Sighs  for regular requests for that to come in. I don't grit my teeth when performing Streets Of London because it's odd what happens to the audience when that comes up. If I don't take the mickey - which I've never done - they respect it. It's flattering that, after 40 years, I'm still getting people asking for one of my old songs because it had had a lasting effect on them.
What are your feelings about the state of folk music today?
‘Young, glamorous and pretty' has entered the equation in the form of, say, Kate Rusby. She also happens to be talented - and there are some incredibly skilled musicians out there. I'd say folk has never been played better. Whether it's better music or not, I don't know, but the social content of the words coupled with music that seems simple will always retain an integrity outside the commercial sphere. Finally, there's something about the acoustic instrument that remains honest - that reason for wanting to pick it up in the first place, even when knowing full well that you're unlikely to make a living out of it.