A little while later and i take back my original assessment of https://github.com/jameswburke/acf-cleaner
It does flag up some items that are still in use (it doesn’t seem to get along with flexible content fields) but if you take care with your field names (and so know which ones are still in use) this seems to be the best solution for now.
You have to delete ONE at a time, and wait for a refresh of the screen which is tedious and if you manually install you need to rename the plugin folder to just “acf-cleaner” or it fails to load the script it needs to work. But with no official solution, this at least works!
