This is what Gabarró's son said:
After recording 'Yesterday' for the first time, McCartney asked Gabby (=Francisco Gabarró) for his opinion (=Gabarró was the principal of the cello section at the London Philharmonic at those days...) Gabby said that, in his opinion, the melody should be recorded in a softer mood, that it had been a bit crisp...
A few days later, Gabarró was in Lleida because his mother had died. He received a telephone call telling that McCartney wanted to record the song again. Gabby said ok, but they will have to wait a few days, because his mother had dead. EMI wanted to do the recording inmediately but McCartney said no until Gabby return back to London. So, they waited for him a re-recorded the song again...
Strange story..., is it not??
Excellent! Thanks for that.
According to Mark Lewisohn's Recording Sessions book, Paul did his two vocal/guitar takes of Yesterday on the evening of Monday 14 June 1965. The book doesn't make it clear whether or not the string quartet was overdubbed during the same session.
However, according to this website ...http://www.beatlesebooks.com/yesterday
... the arrangement was written by Paul and George Martin on Tuesday 15th and the string session took place on Thursday 17th.
Lewisohn says that the song was mixed on that same date - Thursday 17 June - so I'm not sure when the second attempt at the strings, when Gabarro had returned from Spain, could have been.
There is a story on that website (and in the Lewisohn book) which says ....
"Paul ran into Francisco Gabarro a week later in the EMI canteen and thought to say something to him. Gabarro recalls, “He came up to me and said, ‘We have a winner with that ‘Yesterday.’ I said, well, good luck! Congratulations!"
Perhaps there was a second session with the strings AFTER the mixing done on 17th June which hasn't been noted by Lewisohn.