A gardener in the Zentralfriedhof, Vienna's huge Central Cemetery, found me sheltering beneath a tree and assumed that I was lost. I had ducked out of the cold, drenching rain to peel apart the soaked pages of a book. The gardener touched my shoulder: 'Beethoven . . . Schubert . . . Brahms . . .' There - that way] In an eloquent gesture that needed no translation, he thrust out a damp arm and pointed the way I should go.

But I had already visited the well- tended plot inhabited by that impressive selection from the Classical Composers' World First XI. Neither was I particularly interested in the final resting places of the tens of thousands of other people buried in the 495-acre cemetery.

Even had I been capable of expressing myself in German, it is doubtful whether the gardener would have believed what I was really up to. I had come to the cemetery in search of somebody who not only isn't buried there but who had never existed outside the cinema world of make-believe.

The book that I was delicately attempting to prise open was the published script of Graham Greene's The Third Man: my reason for visiting the cemetery was to locate the long tree-lined avenue in the Zentralfriedhof where in the key moment at the end of the film, Holly Martins, played by Joseph Cotten, waits for the Alida Valli character, Anna. But I could find nowhere in the cemetery that accurately matched the movie still reproduced in my book.

Everywhere there were tree-lined avenues that vanished into the horizon, just like the photograph: but nowhere that vanished in quite the same way.

Behind the lines of trees were rank upon rank of the most handsome tombs. Some were fashioned as small houses, others resembling miniature baroque palaces.

Here beneath this magnificent gleaming slab, for example, lay the Familie Nagel, there in greater splendour reposed the Familie Heinisch. The Viennese are buried as they live: in neat, well ordered rows displaying large measures of arrogance and well-bred wealth. It gave me the creeps. A Viennese man said to me later: 'The Central Cemetery is . . . what is the English word you have? . . . we don't have the word in German . . . yes: sinister. It is sinister, don't you think?' Yes, the cemetery is sinister.

For anyone wishing to pursue the trail of The Third Man,it is the perfect place to start.

IN FEBRUARY 1948 Graham Greene flew to Vienna in the hope of finding inspiration for a screenplay. The only start he had was an opening paragraph for a story that he had long before scribbled on the back of an envelope: 'I had paid my last farewell to Harry a week ago, when his coffin was lowered into the frozen February ground, so that it was with incredulity that I saw him pass by, without a sign of recognition, among the host of strangers in the Strand.'

Greene's trip to Vienna had been organised by film producer Alexander Korda, who thought the Austrian capital - heavily damaged in the war and at that time governed jointly by Britain, France, the US and Russia - would provide an excellent setting for a film.

Through his well-placed contacts Korda had managed to get Greene a room at the Sacher Hotel, which at that time was not operating as a hotel but had been requisitioned for the use of British officers. For nearly two weeks an increasingly desperate Greene toured the city's sights from the Central Cemetery to the famous Big Wheel in the rubble-strewn Prater amusement park. But while the city undoubtedly had plenty of sinister ambience he found nothing upon which to build his story of Harry and his mysterious rise from the grave.

On his penultimate day in Vienna, Greene had lunch with a British intelligence officer who told him that when the British had taken control in the city, he had seen a list of Viennese law officers which included a section labelled 'Underground Police'.

'Get rid of these men,' the British intelligence officer commanded: 'Things have changed now.' A month later he was furious to discover that the 'Underground Police' were still on the list. 'It was then explained to him that the 'underground police' were not secret police, but police who literally worked underground along the enormous system of sewers,' wrote Greene. At the same lunch the officer also told Greene about the black-market trade in penicillin which racketeers diluted, with often fatal consequences for those treated with the drug.

Here were the basic elements of his story. 'I had my film,' recalled Greene later in his autobiography, A Sort of Life, remembering the moment with justifiable satisfaction. In Greene's script the resurrected Harry Lime turned out to be a penicillin racketeer who had faked his death to avoid imminent capture by the authorities. Lime continues to evade arrest by moving around Vienna underground through an intricate network of sewers. The story effectively begins and ends with Lime's burial in the Central Cemetery: the first time faked, the second time for real.

DR BRIGITTE Timmermann is anxious. She has just heard of a plan to shoot a film about the making of The Third Man: '. . . with somebody called Robbie Coltrane playing Orson Welles - who is this Robbie Coltrane?'. The planned film worries her on two counts. Firstly, she fears that it will probably be a bad film ('I want nothing to spoil people's memories of The Third Man') and secondly she has been planning to write a book about the making of The Third Man for as long as she can remember - and now she may be overtaken by events.

Dr Timmermann, a guide with the Vienna Tourist Board, loves The Third Man with an affection that borders on the obsessive. For the past six years she has run her 'In the footsteps of The Third Man' walking tour of Vienna, sometimes as often as six times a week. 'On average I get 20 people a tour.' She says that the film, which is shown daily in Vienna's Burgkino during the summer, still continues to exert a powerful fascination.

'People want to see the film and they want to see where it was filmed. It was one of those films like Casablanca which turned out to be much more than the sum of its parts. While it was being made nobody guessed the impact it would have.'

But with a cast that included Orson Welles and Joseph Cotten - working together for the first time since the phenomenal success of Citzen Kane and The Magnificent Ambersons - as well as Trevor Howard and bit-part players such as Wilfrid Hyde White and Bernard Lee, not to mention a script by Graham Greene and direction by Carol Reed, The Third Man obviously had a lot going for it.

'However, luck certainly played a large part,' says Dr Timmermann. 'After filming one night, Carol Reed went with some people for a drink to a cafe in Sievering, a village just outside Vienna. There he saw somebody playing a zither, an instrument he'd never heard before.' The man playing the zither was Anton Karas: Reed had stumbled upon the answer to the problem of finding an appropriate musical soundtrack to the film. 'Karas couldn't read or write music. Carol Reed had to fly him to London, and in the studios he simply played along to the film and they recorded him.'

Reed told Karas of Greene's idea of a theme tune for the central character. Karas shyly offered the deung- de-deung-de-deung tune he had invented years before. Karas's 'Harry Lime Theme' became a huge international hit, topping the charts in America and earning him enough money to buy his own Sievering restaurant. 'He made a lot of money. He died in the 1980s.'

Dr Timmermann's knowledge of Third Man facts and figures is encyclopedic. I asked her if she knew what had become of Herbert Halbik, the dough-faced boy who points out Martins as a murderer to the crowd standing in front of Lime's apartment block. 'Oh, that's a tragic story. He was the son of a lighting technician, it was his only film. When he was 12 he dived into the old Danube, broke his neck and was paralysed. He runs a newsagents now in Vienna.'

The actors, even in the smallest roles, were perfect, the music was absolutely right, even down to the stylised cinematography - frequently shot at a crooked angle - which presented Vienna as a city of black shadows and glistening streets. The lighting was always artfully managed: most memorable is the luminescent glow of Welles's face whenever it emerges from the shadows. 'It was all just perfect,' concludes Dr Timmermann. But most of all, perhaps, there were the sewers . . .

In the brochure advertising her Third Man walk, people are asked to bring along a tram ticket and a torch. The tram ticket is for a rapid transit across the city, the torch is for the sewers - inevitably the starting point.

The regular tour begins at the underground station at Friedensbrucke, where a huge overflow sewer runs into the Danube canal. 'If you hear screaming it's a rat,' she says as we walk into the blackness. 'It's said there are two and a half million of them in the sewers beneath Vienna. And really, they do scream.'

By special dispensation, she also took me down into the sewer beneath Karlsplatz where the most famous Third Man sewer shots were filmed. A uniformed man accompanied us. 'Is he an Underground Policeman?' I asked. 'Oh no, there are no more Underground Police; he works for the council.' The council man was amused at my interest in The Third Man. 'He knows it is impossible for people to get around the city by walking through the sewers. The whole film is built on an impossibility - a myth.'

Carefully arranged camera angles and imaginitive editing succeeded in creating the impression that the sewers stretch on forever. In fact the stairs down from the Karlplatz (down which Harry Lime races in his doomed escape) lead nowhere except to overflows and the underground Wien river.

The next stop on the tour, the doorway across the road from Anna's apartment in which Harry Lime is first seen by Holly Martins, also reveals liberties with the truth taken by Carol Reed. The doorway is there on the cobbled alley running up from Schrey Vogel gasse - just as in the film - but it is a full 10-minute walk from the building used as Anna's apartment block on Am Hof.

'Sure, they cheated, but why not - it's a film.' What is surprising is that for a time when film production companies tried to do nearly everything in a studio to save money, so much of real Vienna was used.

And this is the compelling thing about The Third Man tour. Much of what you see in the film can still be seen in the city. Harry Lime's apartment block remains the same, opposite the Hofburg palace (now being restored after last year's fire) and facing the Franz Josef statue where Harry Lime was said to have died after his 'accident'. The block is a private residence, but I pushed the door open and walked in. And there were the stairs on which Holly Martins was told by the porter that he had missed the funeral party which had just left for the cemetery.

After Dr Timmerman's city tour, my final Third Man excursion took me three stops north of Stephansplatz on the U1 underground line to Praterstern. Five minutes' walk from the station and I was standing beneath the Riesenrad, the Great Wheel where Holly famously confronted Harry Lime.

As a last act of homage, I stood the near the foot of the wheel and got out my damp script and, ignoring the strange looks of the old man emptying the rubbish bins, I read the words that Welles had improvised himself during the filming and had delivered on this very spot: '. . . In Italy for 30 years under the Borgias they had warfare, terror, murder, bloodshed - they produced Michelangelo, Leonardo da Vinci and the Renaissance. In Switzerland they had brotherly love, 500 years of democracy and peace, and what did that produce . . ? The cuckoo clock. So long, Holly.'

A happy pilgrim, I strolled back to the underground station with only one tune on my lips: deung-de- deung-de-deung. So long, Holly.