Thanks Elliot. Sorry if I wasn’t clear… but I wasn’t getting any ACF fields showing on the page I added with “acf_add_options_sub_page”. The name shown in the ‘location dropdown’ is “Site Options” which is the name I gave the sub menu page.
I’m happy to do it another way altogether if that’s better – the core thing I am trying to achieve is to have an ACF submenu page and a standard WP submenu page under the same menu.