Sorry, I should have made it clear that I was not connecting "illogic" to "ill logic". I simply meant in the original post that, when spoken, the two sound very similar. I also should have pointed out that I have not heard "ill logic" used since about 1987 when my A Level English teacher used to use it quite regularly. I too wouldn't encourage its use. "Flawed logic" is indeed much better.
I shouldn't post when I haven't had much sleep!
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