How do I get my user customisable power settings back? I don't use workspaces - don't really have any need for them - and don't want to have to keep switching to change my power settings.

What exaclty do you want to customise ?

When we changed to a more adaptative power management we took into account all use cases, so if you tell me exactly what you want to do I should be able to help.

I want to be able to stop the power saving features being activated while watching videos or copying, up or downloading files and so on. I know I can set up an activity with an appropriate power profile, but I do not always know in advance how long something will take, if I am going to be called away and so on, so I am not always using the correct activity. I have never needed to use the activities before now, and cannot really see the point of them. If I could have different panels in different activities, perhaps.

I used to be able to just click the battery monitor icon and change to my customised profile. Now I cannot.

For this usecase we implemented the following:
In the battery plasmoid yo should see a checkbox or a switch that will allow you to quickly disable any kind of power management, and enable it again once you want.

Ideally all these tasks you mentioned should inhibit the system, we can do this in KDE software super easy, but other software may not ever integrate (which is precisely why we implemented the quick-access checkbox).