30ac (not 20) - The wordplay starts with 'of', which is A ( the Chambers example is Thomas a Kempe) then BUN then A(LIGHT).
I've only just worked it out since until now I've been thinking 'low-sugar' as LITE.
Mersault, I think your description of this puzzle is a hundred per cent bang on. I really admire the work that must have gone in to it but - wow - that was hard.