I'll stick my neck out and make a projection .....
1. As already stated, article 50 is NOT invoked by Mr Cameron, he will allow his replacement to do that.
2. Informal talks take place with EU at very high level
3. After new PM appointed EU (Merkel) offer substantially revised terms of membership to UK (probably with a fudge on immigration/free movement)
4. New PM decides that the new terms might be acceptable to UK citizens and calls another referendum on that basis, without invoking article 50
5. UK votes to remain on the basis of the "better" terms.
Thus,
a) Mr Cameron's statement that he will not have a second referendum is still true .. it is not his call
b) UK stays part of EU which many want (including Merkel)
c) UK terms drastically (?) improved.
d) Leave campaigners whinge but lose
e) Sturgeon claims she can walk on water