So in the middle of recording a video to explain this I figured it out. The top level object had its selection icon unchecked so that’s why it didn’t work!

And I didn’t realise local view worked with whatever is selected… that’s really helpful. Now I can do achieve what I originally needed to do although I still need to remember that slash only works once the cursor is in the Viewport – that’s still catching me out
I will now go back and try and follow the instructions Dwayne posted