On the surface it might seem like arrogance. “I tried to imagine myself in Mozart’s head, to build up a picture of what he would have done,” says Dominic Nunns. “I wrote it as Mozart could have finished it.”

This 23-year-old took it upon himself to finish the incomplete fragment of a concerto composed by arguably the greatest musical genius who ever lived. But Mr Nunns’s completed score of Mozart’s would-be fifth horn concerto in E Major has been widely praised by the experts, and at the end of last year he was awarded the ultimate seal of approval when it was published by the German company BArenreiter, the world’s leading publisher of Mozart’s music.

Mr Nunns was just 19 when he completed the work as an exercise while doing his music degree at Durham university. “We were studying great works that had been left unfinished and completed by other people - Mozart’s Requiem being probably the best-known example. I became fascinated by the idea, and then I remembered that I had heard a recording of a fragment of a horn concerto that Mozart never finished.

“I listened to it again and began to wonder how he might have completed it. People might find it surprising that I could presume to finish off the work of someone as great as Mozart, but he had techniques which he used and re-used and on which you could style the work. And we have all the main ideas set out in the bars he left us.”

Mozart wrote about a third of the now embellished work, with Nunns contributing the rest. “I thought hard about the length of the movement and compared the proportions to his other works. I didn’t want it to be just a technical exercise and I didn’t hesitate to use my own imagination, but neither did I want to stretch that too far.

“One of the reasons Mozart was such a genius was that he tended to break the rules, but I didn’t feel in a position to guess where he might have done that. I thought that would move the piece too far into the realm of speculation. I composed as I thought he would on an average day, sticking to his own rules.”

Mr Nunns’s tutors awarded the piece the exceptional mark of 80 - as good as it gets, for he was later told that it was exactly what Mozart himself would have been given. “Durham’s music department doesn’t recognise marks between 81 and 100, no matter who you are,” Nunns says ruefully. He got a first, at least.

After his course he put the work to one side, but a year later he began to contact potential publishers. “BArenreiter showed an interest. It took about two years, but they eventually decided to go with it, which was exciting because they have a reputation as the people who produce the most authentic editions of both Mozart and Beethoven’s work.” The work has been performed twice (including once at the British Horn Society festival), and the reception has been highly favourable.

“Anything like this tends to stir up a bit of controversy,” says Mr Nunns. “There’s one German professor who is against the idea and has told me so in no uncertain terms. But the feedback so far suggests most people think it is an interesting piece of work and quite realistic.”

He has various theories about why Mozart never finished the concerto. He concedes it is possible the composer thought the piece was no good, but given the quality of the fragment he says tis is unlikely. “I think there is enough there to say that if he had finished it, this would have been the most interesting and best of the horn concertos.”

Started in 1786, when - as ever - Mozart was in financial difficulties, it is more likely that the composer wasn’t being paid for the piece and so set it aside when a more profitable commission came in. In addition, Mozart was never a big fan of the horn, which at that time was a very different instrument, without valves, which limited the notes it could play. On one of his original manuscripts, he wrote “this note will torture your testicles” over the horn part.

“He didn’t take the horn very seriously, although he wrote four concertos which underpin the entire solo horn repertoire. He probably believed he was wasting his inspiration writing for an instrument he didn’t like when he could have been writing for piano or something else,” his collaborator concludes two centuries later.

Mr Nunns learned to play the horn at Downlands comprehensive in Hassocks, West Sussex, under the tuition of Simon Grocott. He then went on to Lewes tertiary college where he did A-levels and a pre-professional music course. After Durham, he spent two years at the Royal College of Music.

Last September he became head of brass at the Palestinian National Conservatory of Music at Birzeit University in Ramallah, on the West Bank. “It sounded like an exciting opportunity. The conservatory has only existed for six or seven years, and although they teach Arab instruments they are aiming to be like a Western conservatory.”

Although he spoke no Arabic when he went, he is now able to conduct lessons in the language. Music, in any case, has a universal language of its own. “It’s slow progress, but it is a case of being patient,” says Mr Nunn. “Ever since I decided to make music my profession, I have wanted to teach and to pass on my enthusiasm.”

The cover of the conservatory’s prospectus features a picture of an eight-year-old Palestinian boy throwing stones at Israeli soldiers outside his home at the al Amari refugee camp. It is superimposed with a picture of a smartly dressed young man playing the viola with intense concentration as a fourth year student at the conservatory. It is the same boy ten years on.

Mr Nunns describes his students, aged between 10 and 18, as “very receptive and talented”, although, like music teachers everywhere, he complains they don’t practise enough.

“We’re trying to set up scholarships to allow some of the poorer kids to come to the conservatory. There are a lot of underprivileged Palestinians who are eager to learn music but can’t afford to do so,” he says.

At present he has only a handful of brass students, but the aim is to have 20 by the end of the academic year. (He has also been filling in teaching piano.) The most exciting development is that this term he will conduct the first ever Palestinian concert orchestra. “The more advanced students are ready to play in a small orchestra, and it’s a great opportunity for the kids who have got to that level. They are going to sit alongside the teachers and we’ll be playing to an audience many of whom will never have heard Western classical music before.”

The score for Mozart’s Concerto Movement in E Major is published by BArenreiter, price pound;6.95. Tel: 01279 828 930 The Palestinian National Conservatory of Music runs a sponsorship programme. It costs pound;650 per year to cover the tuition of one student. Further information from Birzeit University, PO Box 14, Birzeit, Palestine. E-mail: suhail@palnet.com; website:www.birzeit.edu