I know this has been asked before but I couldn't find the answer - is it best to uninstall the old version of CCleaner before installing a new version or best to just install it over the top?

No need to uninstall. Simply install as it will overwrite the files.

It is always best to uninstall something before installing a new version, however, it currently works if you install CCleaner on top of itself. If CCleaner ever comes with less files than it currently does now, then for cleanliness, it'd be recommended to uninstall and then install the new version.