Before \starttext put:

\define[2]\hymn{\bf Hymn #1 \hspace[big] \it #2}\par}

After \starttext put stuff like

\hymn{34}{We three kings}

or

\hymn{10}{In the bleak midwinter}

etc.

This does near enough what I am after: creates a template style command, and replicates formatting in toc.

No doubt, there might be tweaks or even cleverer ways to improve on my efforts.

Thanks to all who took the trouble to read, and consider; and especially Hraban, who took the trouble to offer a suggestion, which inspired me to think more clearly.

Thanks,

Calum