The message
from Derek Turner contains these words:

Is there an organic way tyo get rid of lesser celandine?
tia

Hand forking to below the root depth as soon as the leaves show, then
handpicking every scrap. It works, but takes time and persistence; any
bits you missed will re-grow and require a repeat effort a few weeks
later *before* they start flowering and seeding.

Don't compost the bits :-)

Janet