Mine's been in again to Renault Macclesfield on Mike Roberts's reccommendation.
3 warranty issues are being dealt with, but they can't find anything wrong with regards to the knocking! :@
This despite screwing the arse off it (mpg had dropped from 30 to 24 when I picked it up, over 3 miles!)
Did you demo the knocking??
I wouldn't worry too much about the MPG, they probably left it ticking over in the workshop all day... but then I can't vouch for them in that respect, I just found they were decent folk and interested in talking and listening, rather than just talking down to you.
I'd be taking the dangerous angle, i.e. what would happen if the car were to fail and you (the dealer) told me there was nothing wrong with it? They'll look harder...