Figured out that if you replace:
$m = new MostWanted();
It gets rid of the annoying PHP-5 error:
Fatal error: Non-static method MostWanted::mostwanted() cannot be called statically in [...]
The only downside is that for me it lists the top-5 twice, for some reason. Oh, that and when I left a comment with the fix for Rich I managed to miss the leading $ in the second line (which is present above).
NB: I’m using this as the widget version, caveat emptor.