I've just realised the announcment box appears below who's online on mobile. Given that its at the top of the desktop theme, surely it should be at or near the top on mobile too? I'm sure I can move it, but as a default position it seems to contradict that of its desktop counterpart.

That's not the "default" position, so to speak. That is just where we have placed the block on this site, and when viewing via mobile, it just so happens that the sidebar gets moved underneath the main content (which, if I recall, was done based on feedback).

On your own sites, you have the option to place the block directly above the main content, and it will display in the same location on mobile as well, in that particular instance.

Hmmm.

That is just where we have placed the block on this site

​You purposefully made this important announcement (the only one on the site that mentions you're running beta) half way down the screen on mobiles? Seems like an odd decision