I am using this:
\override NonMusicalPaperColumn.line-break-permission = ##f
\override NonMusicalPaperColumn.page-break-permission = ##f
Which seems to be the cause.
This may yet again come down just to my ignorance. I have used the above construct to stop pages breaking unless I explicitly ask for it. Is this entirely wrong in terms of how to control that?
I readliy confess I do not really know what a non musical paper column is.