Quote - This is an image showing what I mean on the arm-bend.
Is that natural or a problem with the mesh?

It's not all that bad but it just looked odd to me, but then I don't know if our arms actually look like that if we bend them the same way...lol.

I know there will be creases and folds of skin, that's only to be expected.
But thought I'd better just ask in case it is something that needs smoothing out. :)

I tried to reproduce the pose you showed.  I have similar result but the problem can be reduced by adjusting the combination of lCollar and lShldr bones.