I cleaned mine up, after it fell off, by using nail varnish remover - with acetone. I also used some very fine sand paper before that!
A kind soul on TM suggested re-fixing it with sticky strips available from Ford. I didn't use those as the auto parts supplier had a windscreen mirror glue kit that was easy to use. The kit instructions included a useful suggestion to mark the point where it was to be glued with a crayon - but on the outside.
I was warned not to use Araldite as it was likely to crack the screen when it flexed.
I should also say that I bought a much smaller stainless steel mirror to replace the whopping great blind spot that was supplied when new.
