best way to handle it is to shoot the dumb user
Ok after playing a little more today I realised this - I was grabbing a closed primitive (Sphere), applying a solidify selection modifier, I was leaving the skin sides box selected on the modifier, with no sides to skin this appears to mess up the way the modifier works as it doesn't create a clean mesh.
Deselecting the skin sides switch appears to make a clean mesh and sort the error.
Next point of investigation is the smooth function. having sorted the issue as noted above, if I have a skinned sphere with a transparent material, with set smooth on it renders black and with set solid it renders ok. It's like setting smooth engages the classic check your normals type error.
Soon as I move from a closed object to something different all is ok, still to check normals are calculated properly on the inner skin, again will update after playing....