This John Lees composition first saw the light of day as long ago as 1968. In an interview with this website, Keith Damone (official biographer of the band with his wife Monika) said Lees wrote "Mocking Bird" while he was living with the parents of his future wife, Olwen. The song is based on a musical phrase from "Pools Of Blue", which he wrote around the same time.
Lees recorded a demo on a tape recorder borrowed from Olwen, and it rapidly became a live favourite, although it wasn't recorded until 1970, augmented by an orchestral arrangement by Robert Godfrey, which unfortunately would result in litigation lasting years and costing the band a small fortune. The single was released in the UK on the Harvest label, March 3, 1971, backed by "Vanessa Simmons".
Suggestion credit: Alexander Baron - London, England, for above 2