Mike, to make the fermata appear at the end of the line and not at the beginning of the next line try using\once \override Score.RehearsalMark #'break-visibility = #begin-of-line-invisiblebefore the \mark.
Cheers, Martín.
On 3. Oct 2020, 04:34 +0200, antlists <antlists@youngman.org.uk>, wrote: