John Major

[John Clarke]

Anyway, what about the chance that a random member of the population would know the moves? Any takers?

Just a bit better than 50/50 I reckon.

It depends what you mean by 'knowing the moves'. Assuming you mean 'how the pieces move' that sounds like a good guess.

What about the chance that a random member of the population would know the en passant rule?

And what's the chance that someone claiming to know the moves has actually been taught one of the numerous varieties of "Korkser" chess? I've encountered one or two of those in my time (though not in any form of serious play), people who insist you can't castle once the king's been checked, that you can only promote to a previously-captured piece, etc, etc. The Oxford Companion to Chess (first pub. 1984) saw fit to include a brief entry on the subject, and I've no doubt that even now quite a few of these players still stick with invincible ignorance to their idea of the rules.

[Stewart Reuben]

This happened in Majorca. Two children were playing chess. En passant arose and it was ignored. I told the children about the rule, sayin you may not know this rule.
The father said, 'Well I've never heard of that.' I replied gently, 'Well I am an international arbiter'. He responded, 'Well that's as maybe. But I bet my friend has never heard of it and he's a very strong player'.
The chilren, being sensible, just got on with their game.
Now of course I would say jokingly, 'Well I wrote the rules.' When I last said this, the response was, 'No, I know better than that. They're 1500 years old.' Perhaps he would have felt less certain had it been about 200 years old, as with poker.
Stewart Reuben