leeneia, the story you refer to is from 'The Labours of Hercules', the story called 'The Arcadian Deer'. I've got the book in front of me now, and here is the line you remember reading:-

"Er - excuse me sir - but it's true isn't it, that you're the detective gentleman - you're Mr Hercules Pwarrit?" He said the name carefully.
Poirot said, "That is so."

I can't fathom this out, because Poirot would be pronounced Pwa-ro, and certainly not Pwarrit. I'm surprised Poirot didn't correct the lad, as usually he was very touchy about this!