YIKES!!
I checked as you suggested, and indeed it says C/N.
What confuses me now is that previously my BA statement definitely showed 20 tier points earned for the LHR-NCL flights in May and June. I don't recall the actual fare paid, but it seemed it approached 125 GBP one way.
I don't know when the points were taken away, but now I have just 2 weeks to get a fourth qualifying flight in, all along thinking that I already HAD the 4 flights on BA metal, and just needed tier points.