Ahh, ok I had already looked in the footer.php but I didn’t really want to be editing core files. I thought the footer had been created with ACF, although I can’t see a sensible reason why that would be. It is certainly an odd choice.
It is looking like I will need to find another solution to add this link.
Going slightly off-topic for a moment. Do you know why deactivating ACF would break the whole site? If I deactivate the plugin I lose everything except a single bar across the top of the page. Everything else is blank white.