At around eight years of age, Horn took up the double bass and was taught the basics by his father, including the concept of playing triads. He then taught himself the bass guitar and became confident in sight-reading music, using guide books and practising on his father's four-string guitar in the spare room of the house. In his early teens, Horn would fill in for his father on the double bass in the Joe Clarke band when he was late for a gig. At school Horn was given a recorder which he picked up with little effort as he already had music knowledge, and performed in the local youth orchestra. His interests soon turned to contemporary rock acts such as the Beatles, the Rolling Stones, and Bob Dylan. At fourteen Horn played electric guitar in his first group, The Outer Limits, named after the 1963 television series of the same name, playing mainly covers by The Kinks.