Article: Lancashire had a lively culture of choral and classical music, with very large numbers of local church choirs from the 17th century, leading to the foundation of local choral societies from the mid-18th century, often particularly focused on performances of the music of Handel and his contemporaries. It also played a major part in the development of brass bands which emerged in the county, particularly in the textile and coalfield areas, in the 19th century. The first open competition for brass bands was held at Manchester in 1853, and continued annually until the 1980s. The vibrant brass band culture of the area made an important contribution to the foundation and staffing of the Hallé Orchestra from 1857, the oldest extant professional orchestra in the United Kingdom. The same local musical tradition produced eminent figures such as Sir William Walton (1902–88), son of an Oldham choirmaster and music teacher, Sir Thomas Beecham (1879–1961), born in St. Helens, who began his career by conducting local orchestras and Alan Rawsthorne (1905–71) born in Haslingden. The conductor David Atherton, co-founder of the London Sinfonietta, was born in Blackpool in 1944. Lancashire also produced more populist figures, such as early musical theatre composer Leslie Stuart (1863–1928), born in Southport, who began his musical career as organist of Salford Cathedral.

Question: What did David Atherton co-found?
the London Sinfonietta