Apply Custom CSS to one page of the Dashboard

hi @duncanamcclean,
Use ‘admin_enqueue_scripts’ action hook to enqueue your style file in wp-admin.
Have a look in https://codex.wordpress.org/Plugin_API/Action_Reference/admin_enqueue_scripts

Then you can add your custom css in your enqueued css file.

You may find it simpler to just add some custom cs that targets the fact that the dashboard > Appearance > menus page has a body element with a class of:
.nav-menus-php