There was no requirement for units to record the third letter (on the other side of the roundel from the 2-letter squadron code) allocated to an aircraft - they are very difficult to pair up with aircraft serial numbers.
Squadrons and stations kept Operations Record Books, which are held in the National Archives
Royal Air Force operations record books 1939-1945 - The National Archives. ORBs for squadrons can be downloaded for a fee. The information recorded depends very much on the person compiling the record (if he was overworked or disinterested you may get very scant information) and the type of unit (bomber operations were planned, so you usually get crew lists, aircraft serial number and letter etc. but fighter squadrons may simply say "Red Section scrambled").
611's ORB may give you a list of aircraft and pilots, but it is something of a lottery. Good luck!