Oof.  In that case I'm going to have to revert a *lot* of my recent changesets.  Still, all a good learning experience.

(For the rest of this post, I'm kind of thinking out loud - don't feel under any obligation to answer my annoying questions, although I'd be most grateful if you do.)

When you say the imported parish boundary data are correct: the real boundaries are (for the most part) smooth curves, and the imported data are some polygonal approximation to those, right?  And it looks like the granularity of the polygonal approximation is about 15 metres: in some cases, that approximation is in itself enough to qualitatively mess up the correspondence with physical features.  Is there anything in particular we're supposed to do about that?

It's tempting to respond to your point by saying "in that case, where a parish boundary is known to coincide with a physical feature and they're currently mismatched in OSM, we should displace the physical feature to match the parish boundary".  But the reason the parish boundaries are known to coincide with physical features is that the County Council has presented its own (definitive) map of the parish boundaries overlaid on an OS map that includes the physical features - so this would be a slightly indirect way of copying the physical features from an OS map, which I believe is a big no-no, isn't it?

Something I've encountered quite frequently is that a parish boundary coincides with one bank of a river or stream or ditch, but that river or stream or ditch is implemented as a zero-width way in OSM.  I was never very satisfied with the way I was handling that, but in the light of what you've told me, the way I was handling it was very wrong.  Is there a policy for that circumstance?