His innocuously titled Trombone Concerto will have its UK premiere in the hands of the orchestra, its young Faroese principal trombonist Davur Juul Magnussen and conductor Miguel Harth-Bedoya, and represents a long-promised triumph for former RSNO trombonist Bryan Free, the culmination of a 15-year journey.

Free retired from the band a few years back, but Glasgow, and its community of big band music lovers, has been crucial to his painstaking reconstruction of Shilkret’s lost work, originally composed for band leader Tommy Dorsey and given its first performance with the New York Symphony Orchestra under legendary conductor Leopold Stokowski in Carnegie Hall.

Free describes Nat Shilkret as a “musical magpie” and he was certainly a remarkably versatile musician. He composed film scores and popular songs, led orchestras that included some of the top players of the day, and was the musical director at RKO studios in Hollywood. But it was through his interest in Dorsey as a trombonist that Free heard about the concerto. He’d thought it odd that, unlike clarinettists Artie Shaw and Benny Goodman, Dorsey had not commissioned and recorded a “serious” orchestral piece, as was the fashion at the time. A sleeve note on an old vinyl disc pointed him toward the concerto and Stokowski and an obsession was born. Eric Hamilton, proprietor of a well-known hi-fi shop in Glasgow’s West End and a fan of big band trombonists and particularly Glenn Miller, was an important ally for contacts as Free began his transatlantic search, sending letters and e-mails to anyone he thought might have access to recordings or musical scores of the lost piece.

The process was frustrating and full of disappointments and dead-ends.

Although the first performance in February 1945 had been at the height of the bobby- soxers fanaticism and more like a pop gig, the relationship between composer, conductor and soloist was not as harmonious as it might have been and Dorsey switched record labels around the same time, which stymied plans to record the piece. When Stokowski directed it with the Hollywood Bowl Orchestra six months later it was with another trombonist, Hoyt Bohannon. Thereafter it was forgotten, and the orchestral score remained unplayed until Free reconstructed it more than 50 years later.

Shilkret, ever the pragmatist, had done his best to keep the work Stokowski commissioned alive. As he delved into the archives, Free uncovered the existence of a wind-band version, a piano reduction of the score and even a version for string quartet and soloist. The Leopold Stokowski Society, the Library of Congress in Washington, and eventually Shilkret’s own grandson Niel Shell all became part of Free’s quest for the lost concerto. The Shilket family home in New York had fallen into a state of disrepair so when parts of the score that had been found in the cellar were eventually forwarded to Free, they had survived both fire and flood and were in a very poor state.

Incredibly, recordings of the original broadcast of the premiere also surfaced in fragments, two movements from collectors in New York and Florida, another in the collection of a big band fan in Glasgow.

Free took all the fragments and invested in a state-of-the-art computer and Sibelius software, which he had to teach himself to use.

No sooner had he completed one part of the re-orchestration than another piece of crucial evidence would surface and the work would have to be revised. He was convinced, however, that he was re-creating a great piece of music that would have secured a place in the repertoire had it not fallen on hard times.

Although commissioned by Stokowski, it acknowledges its first soloist immediately with a quotation from Dorsey’s signature tune, I’m Getting Sentimental Over You. The work is studded with popular music wit, but it is a proper classical concerto in form, using jazz and dance styles in the same way as Gershwin had done in Rhapsody in Blue and his Piano Concerto. It is also a virtuoso piece. Free has restored Shilkret’s first movement cadenza, which Dorsey declined to play, because it required a technique that was not in his repertoire. Playing “multiphonics” on a trombone means singing a note at the same time as playing another, with the combined resonance producing a third tone, effectively creating an instrument capable of playing chords.

Magnussen, whose own journey from the brass band heritage of his native Faroes to the front desk at the RSNO while he was still a student at the RSAMD is a tale in itself, is a player up to the task. He has, he reveals, bought a new trombone specifically to play the piece, the vocabulary of legato and vibrato required being some distance from that usually asked of an orchestral trombonist. It will, however, be familiar to most fans of popular song as it is widely acknowledged that Frank Sinatra learned his distinctive phrasing by listening to Tommy Dorsey when he was the young vocalist with the Dorsey band.

“The piece would have been a staple of the repertoire if they’d done it better back then,” says Magnussen. “It has not been lost because it is bad. It has surprises all through it.”

He says he will be putting his own stamp on the performance and he is getting every encouragement to do so from Free, who has his own opinions about the performances there have been of the work since he re- constructed it, now totalling 60 concerts. Other people have been crucial to its revival, including former RSNO assistant conductor Marin Alsop, who is now at Baltimore, and American trombonist Jim Pugh, who had long known of the work but has confessed to lacking Free’s “tenacity and perseverance” to rebuild it.

It was Pugh who “re-premiered” the piece in January 2003 in Carnegie Hall with the New York Pops Orchestra under Skitch Henderson. He has since taken it across North America and Europe and made what Free considers the best recording.

The work has not gone entirely unheard in Glasgow, where its rebirth was conceived, with the wind band version being played in 2004 with the Scottish National Jazz Orchestra’s Chris Grieve as soloist and two years later by students at the RSAMD. There is a suggestion, however, that the orchestral version has been waiting for the right soloist to find its way into the RSNO’s concert programme. Free, who cheerfully admits he has made a score that he himself is unable to perform, seems confident he has found the right man in Magnussen.

Nat Shilkret’s Trombone Concerto is part of the RSNO’s Postcards from the Americas at the Usher Hall, Edinburgh on Friday and Glasgow Royal Concert Hall on Saturday.