Brilliant! Thanks barb, that's the one, but my folder structure is different. The instructions say...

Go to this folder: Volume > Users > username > Library > Preferences > Adobe InDesign > Version > en_US > Workspaces and you should have an .xml file with a filename that ends in "Workspace.xml" - open it with textEdit.
Find the line that looks like this: <app-container-control-bar id="31" origin="-15 -2" size="587 32" is-closed="false"/>
and change the "false" to "true" and then save the file.
...but (for anyone else happening across this) in my ~/Preferences I have a folder, "Adobe InDesign" and in it are folders "Version 15" and "Version 16".
(I have a folder, "Adobe InDesign 2021" but the only thing in it is another folder called "16010" containing "CrashModule.txt")
I have edited the corresponding file in the folder "Version 16" and it seems to have worked beautifully. Again tho, the instructions seem to be out of date slightly. I edited the line that reads...
<app-container-control-bar id="4" origin="58 35" size="515 32" is-closed="false"/>
and made it...
<app-container-control-bar id="4" origin="58 35" size="515 32" is-closed="true"/>
Many thanks for the help.
K.