It seems I can't amend a modal page from forms which have been created from a custom template?
How can this be resolved?
Templates control the Maestro Extension Points – which parts of a form can the form designer alter. The default Modal Pages are by default, NOT Extension Points. Depending on your use case, you can either create new Modal Pages to hold your custom content, or ask your template designer to add the existing Modal Pages as extension points in your custom template.