In the spirit of openness and honesty, after a very friendly and helpful contact with Udemy support I can report my issues were most likely all of my own creation!
Advice for others on Mac Air’s - don’t just close that lid and keep suspending everything for weeks on end because it’s convenient - clear that cache and restart your browser and computer once in a while!
Why this affected Udemy in particular who knowws, but I for one can’t really blame them anymore. I hope others get their issues sorted so easily.
P