Hello Werner,
My requirement now solved: I downloaded a redistributable of the SMuFL font Bravura from the official github repo. It has more accidentals than I dreamed of existing. The lilypond ones are 'Stein-Zimmerman' (whatever that is!) and semisharp and semiflat are U+E280 and U+E282. Exactly right. This means I can now refer to these symbols in text documents such as the preface to the string quartet I am doing.
I'm deeply grateful for your guidance.
Andrew