Is there a way to order views grouped fields?
How I resolved this
I don't know what I did wrong in the first place here, as after having removed sorts from my view and added them again the ordering is fine, and not complicated at all
Summary
If you are contructing a view of nodes, grouped by taxonomy term, and using the grouped field (term:title) as views accordion headers, then you can easily sort the accordion headers by taxonomy title/weight, and their child items by node title. To do this add a first sort criteria for term:title/weight(asc), and a second sort criteria for Content:title(asc).
I opted for a sort based on weight in the end, as I can always reset vocabulary weights to alphabetical order, and this allows the editors a bit of free choice if they need to prioritise the list.