Hi

> Presumably something is converting it from 8bit to base64 along the
> path.

If it makes a difference my setup is gmail->offlineimap->notmuch

> I scouted around for other messages that include © (which is what I
> presume causes the encoding), but the ones I have are all still 8bit.

A different example which shows the same problem (for me) is
id:"m2fwg1uapw.fsf@gmail.com"

Many thanks

Mark