@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:

https://wiki.openstreetmap.org/wiki/Tag:highway%3Dtertiary
"...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."

https://wiki.openstreetmap.org/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.