My plan was to leave the old ceiling rose in place, and just mount the other ceiling rose cover over the top. I could just screw two brackets into the ceiling on either side of the ceiling rose, and fix the cover to that. And it worked! It doesn't look immaculate, but hey, what does.
This was the bracket the lamp was supposed to hang from. But you can't put that up over the ceiling rose! So I decided to only use two right-angled brackets, one on either side of the ceiling rose.
It works! The bracket you can see doesn't reach to the other side
The almost-final result
The one thing I need to sort out is that the three wires are height-adjustable; I'm not yet happy with the relative height I have them at. I will have to tweak that! But the structure is sorted, and the amount of light this set-up produces is amazing. Mission as good as accomplished!