Hi @Tim_Peppiatt! No worries, I’m happy to shed some light on this issue! Your Premium plan only applies to your Organization and do not cover your Personal Project, that explains why you don’t have access to custom fields in this space. As @Bastien_Siebman mentioned, Personal Projects are not really flexible; as an example, they don’t support premium features. In fact, in the future we’re planning to depreciate them (you can learn more about it in Why maintain Personal Projects? - #5 by Alexis). Instead, I would suggest to create a brand new Workspace (private to yourself) and to transfer all your tasks using Kothar. In order to use custom fields for this Workspace you would need a premium plan, but you might want to look into tags instead.
Hope this helps! Let us know if you have any follow-up questions!