Is it of any significance that Margaret Gelling omits Quarrendon from her list of cweorn-dûn names?  (Landscape of PNs, p169: Quarlton, Quarndon, Quarrington, Quorndon).   Is the extra -e- (cweoren) so common in the early forms (PN Bucks 137) a reason for doubting that the word is cweorn?
 
Keith