Mark Burton wrote:

That northeasterly-bound section of B4290 isn't a single way because of 
bus-route issues, but I made a "through_route" relation to span across 
the junction "just in case". It made no difference.


But as a left turn isn't actually allowed there, perhaps the GPS doesn't
bother to tell you to turn right as it "knows" you can't turn left?

Don't think so.
On a different junction last night I got told to turn left on joining a dual-carriageway from a side road when there was no alternative.
Steve