OK, done. I undid it with a combination of JOSM's "reverter" plugin and the Perl revert scripts.
JOSM is an OSM editor that is an alternative to the in-browser one - see osm.wiki/JOSM . It supports plugins, such as osm.wiki/JOSM/Plugins/Reverter which can be used to revert changesets.
The perl scripts are described at osm.wiki/Perl_Scripts and allow programmatic object by object changes.
In iD (the in browser editor) if something has gone a bit wrong, such as an accidental node drag, you should just be able to "undo" back past the problem and then go forward again.

I'll have a go at undoing it

Thanks!

Thanks - fixed

Er, "contact me in private message for souces" isn't really OK. Please include source details here.

Oops - forgot to close changeset. There's nothing other than in the very northeast and the very southwest, honest.

Thanks - I'll sort that out.

@Pink Duck but it sounds like what you're saying is that in your example the "highway" tag was wrong - it's not just for foot traffic, and so DaveF's "fix" here is incorrect (and has actually made the problem worse)?
The southern part of way/992713995 can't logically be a highway=footway and the aerial imagery suggests not either, but I'm not familiar with the area of course.

@DaveF re "you may thank me" above, being a smug git is not helping anyone, especially you. Let's try and work together here and understand each other's point of view.

Thanks - I was wondering what the "new national park" on https://map.atownsend.org.uk was!

One situation where "Remove access=private from footways with designation=public_footpath" might be incorrect is where the it's not the access tags that are incorrect but the highway tag. If any of these 44 ways was actually a highway=track, but incorrectly tagged as a highway=footway, then it's the "highway" tag that needs changing, not the "access" tag.

For completeness, https://overpass-turbo.eu/s/1lOm is a shared query that is close to what you might want. All of these are likely mistagged, as the "access=private" on a footway will be overridden by the "foot" tag, and access tags for other modes are assumed to be "no" by default as it's a footway.

in the case of way/992713995/history , not really - that public footpath (with "foot=designated") has been there since you added it 11 months ago. The "access=private" never had any effect, because it was "highway=footway".

@DaveD Does OutdoorActive route down "highway=footway; foot=yes; access=private"? If it doesn't, that's a bug, and it'd be great if you could badger them to fix it. If not, I'd suggest stopping using that app because it is just broken.

@Pink Duck Can you please give an example of a way that was adversely affected by this change where mapping that was correct "on the ground" is now incorrect?
So far I'm aware of basically two sorts of cases in the list below - ones like way/723570831/history where the original access=private was surely a cockup, - something looked private, but technical actually isn't, and ones like way/1054074314/history which is a different sort of cockup - a "private" tag looks like it got left over from splitting the way. Maybe the error in this second one is with the highway tag not the access one, but either way I don't see how information is really lost here.

Hello,
Just to let you know, I've joined up the North Cheshire Way here like this:
relation/63113#map=18/53.28762/-2.72493
I hope that is correct.
If you're familiar with the area, perhaps you know how it should get across some of the other gaps that can be seen at http://ra.osmsurround.org/analyzeMap?relationId=63113 ?
Best Regards,
Andy

Hello,
Just wondered if the deletion of Derryduff More townland here was deliberate (you can see where it was at https://overpass-turbo.eu/s/1lMC ). The changeset description just says "updated" so it's difficult to tell whether it shouldn't have been there in the first place or was deleted by accident.
Best Regards,
Andy

Hello,
Is protect_class=2 correct for relation/10609122 ? Normally that's used for National Parks, I think: protect%20class=2?uselang=en-GB .
Cheers,
Andy

Thanks.
Since there has been conflict about names here previously, perhaps you could add a few words here explaining why the names here are correct?
It would give us something to refer to in future.

Hello HKHikerhang,

You've deleted the peak node/8781290893/history . This seems odd - peaks don't normally disappear! How does that relate to the changeset comment here 'Change to "old_name" for those names not commonly use nowadays'?
Best Regards,
Andy