I found this:
"In the UK film Alfie (1966, UK), Michael Caine (as the title character) said a lot of phrases such as:
"(You) know what?"
"(You) Know what I mean?"
"(Do) you know..."
"I don't know..."
"...I don't want to (wanna) know."
However, Alfie never said: "(And) not a lot of people know that" - although the catchphrase (in many variations) was attributed to him for many decades.
Caine claimed that it was actually Peter Sellers who had impersonated his voice with the phrase on his telephone answering machine: ("This is Michael Caine, Peter Sellers is out. Not many people know that"). Sellers repeated the humorous phrase during an appearance and impersonation of Michael Caine on BBC-TV's The Michael Parkinson Show in 1972.
"It takes a man in a tweed suit five and a half seconds to fall from the top of Big Ben to the ground. Now there's not many people know that!"
The phrase was also given to Caine (as the character of Dr. Frank Bryant) as an in-joke in the film Educating Rita (1983, UK), who drunkenly spoke:
"Did you know that Macbeth was a maggoty apple? Not many people know that!"
"
From here 'Greatest film misquotes':
https://www.filmsite.org/moments02.html
But since it was funny in any case, here is this Michael Caine impression compilation, certainly a legend in everyone's mind - and in this one even his own:
https://m.youtube.com/watch?v=-qIy1DmGLWI
