"I'm not in love" was not a Mellotron. Neither (strictly speaking) was it a 24-track machine running in sync. The notes were not "punched in to extend them".
Other than that, you're close. Very close.
It was recorded at the Band's Strawberry Studios in Stockport. The console was a Bright red wrap-around Helios (a picture of which is actually on Dan Alexander's website, -the exact console) and the tape machine was a Studer A80 2" 24-track. The notes were sung one at a time,perhaps ten seconds long, about as long as the note could comfortably be held (multitracked, 24 of each) onto 24 tracks, then mixed to 1/4". Then the next note was done. When all the notes were mixed down to 1/4", the mixed "submasters" were cut into loops. The loops were various in length, perhaps some were 7-point-something seconds long, others might be eleven seconds... who knows, who really cares! Where splice points sounded in any way too obvious, the tape was cut and the edit was re-tried, until a better sounding loop point was found.
Next the loops were loaded one at a time onto the 1/4" machine. They were played back, loop points happening wherever they randomly ocurred, and each was recorded onto a blank piece of 2" tape, each note onto a different track.
The resulting "Mega-loop-multitrack" was played back and the faders formed the "notes" for the "players" to 'play' the voice "aah" loops back into the tune, on the "real" multitrack tape.
There was no synchronisation. None was needed. There is no timing information contained in the "aah" reel, and if it ever ran out, or if any of the "playing" was late, early, or just plain wrong notes, the 'proper' multitrack could be rewound and a punch-in made, without ever having to rewind or re-sync the "aah" reel. Similarly, if the "aah" reel ever ran out, it could be rewound to the start, and the multitrack tape backed up a few seconds, and a punch-in ("drop-in" was the term in use at the time) made, then carry on as before.
The idea for the whispered voice in the Middle section:
"Be quiet... big boys don't cry....
big boys don't cry....
big boys don't cry....
big boys don't cry...." etc...
was from the receptionist, who came in while they were listening during the recording part of the session, while they were looking for an idea to put in there. She didn't want to interrupt, so she went up to Graham ( I think it was Graham, anyway..) and whispered to him that there was a phone call for him. -The sound of her whispering gave him the idea to put her in front of the mic, and from that moment forward, she had a place in music history!
Anyhow, just wanted to put the record straight.
That helios was a pretty individual thing! -It even had a cigarette lighter (12V, out of a car!) in it!
Keith