I've reverted the construction changes and reached out to the Magic Earth team, I'll let you know if they get back to me.

Magic Earth actually hasn't updated it's OSM data yet so I was still able to replicate the old behaviour. It handles that way you linked fine, perhaps because of the longer junction arms?

I don't really like editing for the router in major cases (like changing an entire road classification), but this seems fine. At the same time though if there's other cases it might just be best bringing it up to the Magic Earth team to see if they can fix it.

It certainly seems strange to me, because surely routing by this junction doubles the length travelled? I do wonder if putting give way nodes would help.

Hey @Paul Berry, I was using Magic Earth when it tried to navigate me off the roundabout and then immediately back on via these ways instead of going around the roundabout as normal. I understand that this is technically "editing for the router" but if it doesn't provide any value keeping it as a normal road then I think it's fine?

You raise a good point, I was thinking of connectivity in terms of routes between villages, but having easy access to a major A road without having to go through another town does mean it makes sense to mark it as a higher priority route. As an aside, although I am used to driving on rural Norfolk roads, this particular one is really bad in terms of potholes and maintenance. Perhaps navigation apps will take into account the smoothness tag when calculating longer routes.

I'll change it back to a tertiary now. Thanks for your help!

@Pink Duck, thanks for taking a look. I imagine you have a lot more experience than me but I made this decision looking at the wiki:

highway=tertiary
"...For the purposes of mapping, it's normally best to tag distributor roads according to their relative importance in the road hierarchy, as described above.

One rule of thumb for UK roads is that highway=tertiary works well for roads wider than 4 metres (13') in width, and for faster or wider minor roads that aren't 'A' or 'B' roads. In the UK, they tend to have dashed lines down the middle, whereas unclassified roads don't."

osm.wiki/Roads_in_the_United_Kingdom
"Generally used only on roads wide enough to allow two cars to pass safely where adequate road markings are in place"

"There are exceptions to the table above where some roads are not mapped by their legal classification because the actual usage of the road is not in keeping with the legal classification."

Is it not correct to therefore tag this as unclassified despite the official C classification, as there are no middle road markings and there is not enough space for cars to pass side by side? I note the above link also states "Country lanes" should be classed as unclassified.