Hi Neil,
are you working on the curves you posted ?
If so I will check what I modified to see if that helps.
I posted about the Karkow method where I used the nodeeditor
And James Retopo nod to produce the facets thus cutting out
having to do the iso lines,convert to polygon and loft manually.
I am getting some good results and some unexplained at the moment
but hope to better understand the process to get consistent results so it can be a useful tool.
Anyway keep persevering.
Barry