Update
Replaced both door receivers yesterday; well, if you're replacing one you might as well do both. And what a difference! The range has doubled maybe almost trebled and of course I haven't had any intermittent problems - yet!!
So - although expensive, so far, it looks good.
BTW - the Barrels+IR receivers (all one unit), when ordered from MB, arrived fully matching my car so no need for different keys
One annoying problem down - only three to go!!!!!!