I recently wrote to John Walker after hearing "The Sun Ain't Gonna Shine Anymore" for the first time in a very long time, asking if he would confirm or dispel that the intro (and outro) was done on a Mellotron MkII using the tenor sax sound. Seeing that the recording was made in England, it is likely that a MkII was available in the studio at the time.
His response in this morning's mail:
"So sorry to have taken so long to reply to your message. I'm in the middle of a UK tour at the moment. No we didn't use a Mellotron Mk11 for 'The Sun'. It does sound as if we did though."