Tucker's crusade began, as so many of these things do, because he got fed up waiting for someone else to conduct the music that he wanted to hear, not least the early Elgar oratorio The Light of Life. In the late Sixties and early Seventies, unfashionable Elgar was hardly the household phenomenon that he has since become; the major pieces were on record, but that was about it. And so in 1971, though Tucker, then 25, had never stood in front of an orchestra before, he got some singers and instrumentalists together and heard his Elgar from closer quarters than he had ever previously imagined. That concert was intended as a one-off, but so many people asked him what he was going to do the next year that it set him thinking.
Since then, the Broadheath Singers and Windsor Sinfonia (Tucker's amateur chorus and ad-hoc professional orchestra) have rescued from oblivion well over a hundred works, most of them unperformed since the early years of the century. Elgar, Parry and Stanford have been mainstays of the programmes, ceding ground as other musicians began to rediscover them; and Tucker's revival (in 1974 and 1984) of two big works by Granville Bantock - Sea Wanderers (1906) and The Time Spirit (1902) - must have helped fuel the critical reassessment of Bantock that is currently gaining momentum.
But the Broadheath agenda has always extended to composers with barely a toe-hold in the dictionaries - people like Alexander Brent Smith, Martin Shaw, Lilian Elkington, Eric Fogg, Inglis Gundry, Ernest MacMillan - names that the average classical-music buff won't recognise, though they were prominent in a golden age of choral music at the end of the 19th and beginning of the 20th centuries, when all of England bristled with choirs, and composers and publishers did good business supplying a buoyant market with attractive, singable works.
Though each of two world wars did its bit to choke back the choral tradition, there are still plenty of local societies which would love to sing this stuff. The trouble is that the music itself has often disappeared. Publishers have gone bust, been swallowed up or moved premises, often junking the scores and parts that they thought no one would ever want to see again. As a result, not everything is ready for revival. Many works survive only in the vocal scores the choristers would have sung from, where the orchestra is reduced to a simple piano outline. But Tucker doesn't let such obstacles get in the way of performance, through the simple expedient of commissioning re-orchestrations that allow the music to be heard again in something like its original form. Thus, for instance, he drafted in composer Rodney Newton to rescue Ernest Farrar's The Blessed Damozel (1907) and Eric Fogg's The Hillside (1921), the full scores of which had vanished from the face of the globe. It's an expensive solution, particularly since Tucker and his small band of supporters operate on a shoestring, with grants and ticket sales rarely covering their costs, but he prefers to put the music first and worry about the money later.
His researches benefit from the active help of Lewis Foreman, not an academic musicologist but a hands-on activist who knows more about the missing music of this period than anyone else does. And their archaeology requires genuine fieldwork, as Tucker explains: "From time to time you'll get a rumour that somebody's attic probably contains such-and-such a piece, and you go looking for it. But invariably you don't find it - you find something else which is far more interesting. Lewis and I went down to search through Montague Phillips's grandson's attic because we were looking for a full score of The Death of Admiral Blake. We didn't find it, but we found a whole load of other stuff by him, with a full score and set of parts for his symphony. You just never know what you're going to find."
Tomorrow's concert offers three choral-and-orchestral pieces, inspired by the turn-of-the-century fashion for setting pre-Raphaelite poetry. The most eagerly awaited item is the world premiere of The Lady of Shalott, for mezzo-soprano, chorus and orchestra, written in 1909 by Cyril Rootham (1875-1938). In 1995 Tucker conducted Rootham's 1915 war-elegy For the Fallen; when he took the parts back to the Rootham archive in St John's College, Cambridge, he came across the full score of The Lady of Shalott and read through it. He couldn't understand why it hadn't been published or performed and he determined to put it on - and Rootham's grandson, who has paid for the score and parts to be typeset on computer, will be singing in the chorus.
For Cecil Armstrong Gibbs's La Belle Dame sans Merci (1928), only half an orchestration was required: the string parts survive in the British Library, and instrumental cues in the piano part have helped the orchestrator William Llewellyn with the rest. Tucker describes the music as "atmospheric, with some quite weird chords; very mysterious, and yet very approachable". Edgar Bainton's setting of Rossetti's The Blessed Damozel for soprano and baritone soloists, chorus and orchestra - written, like Farrar's, in 1907 - posed a different sort of problem. The full score and orchestral parts did exist, but in the inconvenient location of Sydney, so that microfilms had to be sent from Australia. "Highly chromatic, very lush - and well worth doing," says Tucker of it.
The closest you get to the mainstream tomorrow evening is Holst's orchestral Elegy (In memoriam William Morris). It was originally the slow movement of his "Cotswolds" Symphony, written exactly 100 years ago and just been released on CD by Classico. But for all the popularity of The Planets this, too, is an extreme rarity on the concert platform.
Tucker's philosophy for choosing the works he does is straightforward: "Although the composers may not be known, I always have to do music that is tuneful, singable and listenable-to. The chorus and I have got months to learn it and get it under our skin, but the audience has only got one chance, so they've got to find it something memorable. And I've got to make at least one attempt to present these things, because I don't see anybody else doing it - nobody's bringing out the music of British composers born between 1848 and 1910 on a regular basis except us."
The amount of music still to get a hearing is enormous: Tucker's efforts have uncovered only the tip of the iceberg. Foreman castigates the approach that other promoters take to such works: "People don't seem to want to perform them unless they are masterpieces, but that's absolutely the wrong way of looking at it. They're all worth at least one performance, and until you've given them one performance, you can't assess them." So what works does he think ought to be heard next? "The big composer who really does deserve a reassessment in performance is Walford Davies. And Havergal Brian's Cleopatra is a purple, Romantic choral work; the full score is gone, but it only needs re-orchestrating and would be really very good."
Time, moreover, is not standing still, as Foreman warns: "Even while Robert's been doing this, things have been disappearing left, right and centre. And when sets of choral parts disappear, you're faced with such a large expense that you can't necessarily revive them." In the meantime, he can cast his mind back over Tucker's track record with evident satisfaction: "In all that time I don't think he has had one serious dud."
School Hall, Eton College, tomorrow, 8pm. Tickets pounds 8 from 01895 847083 or on the door