The Ending (Spoilers)
I very much enjoyed Summerfield - something of an unsung gem and excellently and atmospherically directed by Ken Hannam.
With regard to the ending though, unless I missed something the point of Flynn turning up was to indicate that Simon shouldn't have put his nose in where it wasn't needed. But all of the events would have panned out pretty much the same if he hadn't investigated - after all he was invited to the Abbott's house for dinner, and he only went back to peek about because his car broke down.
Also why did Flynn have that photo of the cannon and write 'saltwater in the blood' on the back of it if he had nothing to do with it?