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Pablo, Excellent. Works like a charm. Many thanks.
From Pablo's suggestion, I replaced my original two 'sectionblockenvironment' blocks with:
\definestructureconversionset [sectionnumbers] [I,N,KR,a,m] [N] \definestructureconversionset [appendix:sectionnumbers] [O,A,R,r,KA] [R] \setupheads [chapter,section,subsection] [sectionconversionset=sectionnumbers] \startsectionblockenvironment [bodypart] \setuphead [chapter] [style={\rm\bf}, sectionsegments=chapter] \setuphead [section] [style={\ss\bf}, sectionsegments=section] \setuphead [subsection] [style={\sl}, sectionsegments=subsection] \stopsectionblockenvironment \startsectionblockenvironment [appendix] \setuphead [chapter] [style={\rm\bf}] \setuphead [section] [style={\sans}] \setuphead [subsection] [style={\sl}] \stopsectionblockenvironment