I have sheepishly avoided posting anything on bird feeding since this fatal illustration of the principle of unintended consequences.
Disposing of the prior feeders (although this may have been overkill) and regularly cleaning, and moving around, the newer ones, seems to have helped.
There have been no more dead greenfinches. This is coming into a quieter time in garden bird feeding anyway. I have seen greenfinches since, but not in the same numbers. The sad thought that this is due to my own doing persists.
A pleasing mix of species, currently dominated by corvids but with a proportionally higher amount of tits than before, still feed.