Thanks for all the responses, this does appear to be a subject that can certainly provided different ideas and concepts all of which to appear correct. It seems that different authors have made slightly different cases to justify their answers.
To keep the ball rolling I still have a issue explaining the idea of spanwise flow having in this case. The reason for this is (as my understaning goes) span wise flow will create wing tip vortices. This in turn creates a downwash rear of the trailing edge. This causes a resultant airflow ahead of the wing to be "slightly angled downwards". With this in mind surely this reduces the effective angle of attack of the wing tip and in a round about way protect it from reaching the critical stall angle prior to the wing root.
All answers so far have been a wealth of knowledge I am just digging further so I dont have to bluff my way thru any questions