Forbes was born in St Andrews, the son of a Scottish country fiddler. At the age of five, he had some lessons from his father and the lady who played in the local cinema orchestra, and later from H. Everett Loothby in Dundee. At 12 he made his first successful appearance in a school concert, but confessed that at the time he was far more interested in the yellow stockings and red garters that he proudly wore with his kilt.
After winning first prize in the Perth Festival, he decided it might be fun to take up music as a career. So, at 16, he was sent to London to study at the Royal Academy of Music, where he had violin lessons from the legendary teacher Editha Knocker, and with Raymond Jeremy on the viola. He played in the Academy Orchestra under its conductor Henry Wood and was also second violin in a student string quartet.
It was quite by accident that he took up the viola. The viola player in the Academy quartet had been offered a job in the newly formed BBC Symphony Orchestra and, viola players being thin on the ground, there was no one to replace him. So Forbes was persuaded to take a viola with him during the summer holidays and on his return, performed so successfully in an Academy concert that he decided he preferred that instrument to the violin.
He subsequently auditioned for the Royal Philharmonic Orchestra for the violin but mentioned that he also played the viola. As a result, he was offered a "gig" at the Queen's Hall and had to play "a terrible part" in the Beethoven Coriolan overture, having to memorise the part because he still could not read the viola clef.
In 1930, Forbes went to Pisek in Czechoslovakia to study with the legendary Otakar Sevcik, whose intricate system of exercises revolutionised string playing; he felt he had benefited enormously from this period of tuition: "Sevcik taught me how to practice and how to tackle difficult passages." Following this concentration on technique, Forbes was fortunate in having some lessons from Albert Sammons, one of the great string players of the time. "He was marvellous. He taught me how to perform - how to put music across to an audience."
At this time, Forbes became a member of the Stratton String Quartet, led by George Stratton, and stayed with them throughout the Thirties. One of Forbes's particularly cherished possession was a signed copy of the famous picture of Elgar on his death-bed listening to the first recording of his Piano Quintet played by the Stratton with the pianist Harriet Cohen, the artists chosen personally by Elgar. In October 1933, Elgar had an operation from which he never recovered and HMV made the recording as a Christmas present to the dying composer.
At the onset of the Second World War, Forbes was joint leader of the London Symphony Orchestra, but from 1940 onwards he joined the famous RAF Orchestra which contained a number of small groups of chamber music players, all famous soloists in their own right. Forbes toured the UK in a piano quintet which included Denis Matthews, Frederick Grinke and James Whitehead. With the exception of Gerald Moore, he also made more appearances than any other musician in Myra Hess's legendary concerts at the National Gallery.
After the war, Forbes returned to Czechoslovakia to appear with the quartet - now renamed the "Aeolian" - in the first International Music Festival in Prague. From this time onwards, Forbes continued to play in a number of small chamber groups and also as a soloist in the concerto repertoire for viola. In 1954, he became professor of viola and chamber music at the Royal Academy of Music in London, served on the music panel of the Arts Council and was also an examiner on the Associated Board, which he gave up only in 1985.
In 1964, Forbes moved to Glasgow to take up the post of Head of Music for the BBC in Scotland. He always felt that this decision had brought his career full circle. He oversaw the BBC Scottish Symphony Orchestra and a number of other organisations connected with broadcasting in his native land, and was Chairman of the Sir James Caird Travelling Musical Scholarships - which had enabled him to go to Sevcik so many years before.
He also found time to organise a viola competition in London in 1969/70 at which William Primrose adjudicated. Forbes's Competition for Scottish Violinists attracted over 150 entrants and Yehudi Menuhin came to judge the finals.
In the late 1960s he directed the Montrose Festival; in 1970 he was made an honorary doctor of music by Glasgow University and in 1972 was awarded the Cobbett Memorial Prize for services to chamber music.
In 1978, he and his second wife, Jean, a professional singer, moved to Warwickshire, and in no time a local string quartet was in evidence. In his latter years, Forbes devoted his time to making arrangements for a number of different instruments. His published works are in demand the world over; in fact when William Primrose first went to Japan, he discovered his fellow Scot was better known than he because the students all played Forbes's arrangements.
Watson Forbes was extremely well read and was not only an entertaining and witty conversationalist but also a good listener. He was admired and respected by his students because he was always positive in his judgements, giving constructive criticism in order not to undermine the individual's confidence. He is survived by his wife Jean, and by his two sons, the composer Sebastian and the singer Rupert.
Watson Douglas Buchanan Forbes, viola player: born St Andrews 16 November 1909; married 1937 Mary Hunt (died 1997; two sons; marriage dissolved), secondly Jean Beckwith (three stepsons); died Moreton-in-Marsh, Gloucestershire 25 June 1997.