What's the Point of Symphonies?

What exactly is a symphony, and how can one written in the 18th century by the ‘father of the symphony’ Joseph Haydn (he wrote over a hundred), have anything in common with one written today? Where did they come from in the first place, and why did they come to dominate classical music for centuries? Why do they still feature in almost every orchestral concert programmed, when so few are actually commissioned? Tom Service investigates with help from our witness, composer Deirdre Gribbin. Producer: Ruth Thomson