So with nothing to lose I took some fine sand paper to them to get back to a smooth, clean white, surface. I started with 180 grit nice and gently, hardly any real pressure needed, then finished with something finer. It was just as easy to get in round the edges near the mirror and ball joint.
To smooth them out further I rubbed/polished them with Plastic Weld (a very fine liquid cement used by modellers). This gently melted the remaining marks left by sanding leaving a smooth and scratch free surface. I then polished them some more with a normal paint cutter/polish and they look so much better. They are white and smooth as you like.
Pic of the second one half done just for an example.
For less than a couple of quid and about an hours work or so I've got mirrors that look nearly new (if you ignore the hole shown in the first one).
