Mad (Flt) Scientist,
I'd draw your attention to my comment <<simple reason understandable to a mere Basil >>. I am an experienced practical pilot, ex mil with commands with two majors.
I am not disputing that you may be able to prove, in theory, that a straight wing aircraft will dutch roll however I have never flown one which does so at the flight levels normally occupied by straight wing aircraft, i.e. not Starfighter etc which had another set of roll/pitch problems.
I was referring to sweep, not dihedral and yes, sweep does cause dutch roll.
Correct me if I'm wrong but the basic reason for dihedral is to improve roll stability.
I do appreciate that many factors contribute more or less to aircraft stability however since the thread was started by a PPRuNer calling themselves 172_driver I'd guess they'd prefer a short rather than longwinded answer.