Had a bit of a breakthrough (possibly) as to why I've been having so much bevel setting trouble - I think my Naniwa 1k has a fairly thick layer of release agent, or something similar.
It is flat, but I got frustrated and took it back to lapping to try and refresh the surface - and it's a totally different stone! The texture, appearance (not to say performance) have changed significantly!
I do feel like a right tit though, don't know how I didn't notice!
Has anyone else come across anything like this?
I vaguely recall someone mentioning something similar on a Norton before, but I may have imagined that.

Anyway - don't lap your stones when your frustrated, go and have a coffee instead... It's not flat any more