Revamping of the scheme is expected soon. See the chapter on Retainees. Assistants are doctors who tend to work regularly for a practice full or part time. Some doctors will be assistants in more than one practice or will be locums at the same time. See the chapter on Assistants.
Associates are few and far between.
They have tended to be in rural areas and are mainly in Scotland. They are a formal grade of non-principals, defined in the Red Book. These are similar to 'official' associates but the posts are created especially for that scheme and so terms and conditions of service will vary. See the section on Associates. A Deputy is really any doctor who is acting for a principal.