In November of 1974, Neil Sedaka released Sedaka's Back on Elton John's Rocket label. The album is actually a compilation of new songs and a few from previous albums including those made at Strawberry Studios with 10cc. The album cracked the Top 30 thanks especially to "Laughter in the Rain, a #1 hit in the US.
In one year Sedaka says he went from making $50,000 a year to $6 million. The comeback was firmed up by the 1975 #1 hit "Bad Blood" in which Elton sang back-up. Other artists found big hits on Sedaka's Back. The Carpenters covered "Solitaire" and The Captain and Tennille took "Love Will Keep Us Together" to #1.
While "Laughter in the Rain" was almost surely recorded with LA session musicians, it isn't clear who performed on the Sedaka tunes. My ears, long associated to listening to 10cc, would bet the humorously macabre track "Little Brother" has 10cc on backing vocals.