The reason for ridges on a pellet skirt.

I'm pretty sure this statement is wrong.
"In brief this means that the spinning of the pellet as it travels produces a low pressure area to the side of the pellet into which it is sucked. That gives a pellet deviation to the side of the direct line from muzzle to target. Left to a clockwise rotating pellet. "

There is no difference between the left or right side (or top and bottom for that matter) when the pellet is rotating.
The magnus effect will change the flight path when the axis of rotation not in the same direction as the air flow over the projectile (eg a crosswind).  See http://en.wikipedia.org/wiki/External_ballistics#Magnus_effect

I hadn't read your last post before I wrote mine. Yep spot on, a side wind will cause an elevation change, how much I have no idea.

I would have guessed ridges on a pellet would make the effect more pronounced, as that is why golf balls are dimpled and not smooth ??

I have wondered why some pellets have ridges, but since most of the good ones don't seem to I haven't bothered trying to find out.