The horn arrangement was done by McCartney & Martin, since McCartney can't read/write music. The picolo trumpet solo was McCartney's idea after watching a performance of Bach's Brandenburg Concertos on the BBC. The actual song (melody, chord sequence, etc.) is McCartney's with a *heavy* Beach Boys/Brian Wilson influence.
No way. Circumstantial proof? Paulie's first solo album recorded not long after that.