Thanks Peter
My javascript console does not have the errors listed in that bug - instead I have:
This page uses the non standard property “zoom”. Consider using calc() in the relevant property values, or using “transform” along with “transform-origin: 0 0”. held_messages Source map error: Error: request failed with status 404 Resource URL: https://downtonband.org.uk/mailman3/static/postorius/libs/popperjs/popper-v1... Source Map URL: popper.min.js.map
Source map error: Error: request failed with status 404 Resource URL: https://downtonband.org.uk/mailman3/static/postorius/libs/bootstrap/js/boots... Source Map URL: bootstrap.min.js.map
I'm also on an older version of the debian packages than listed in that bug... still I tried the suggested fix just in case but it did not help.
Thanks for the suggestion though - it certainly looks similar.