I’m doing my CV. I worked for a year as a Legal Case Worker but then (due to poor performance) was demoted to a Legal Secretary. So, for the time at that firm do I put “Legal Case Worker demoted to Legal Secretary” or just put “Legal Secretary” to cover the whole period of employment?
I would put "Legal Case worker" and detail that role, with dates, then from the date your job changed, put "Legal Secretary" etc. If you are questioned about the change of role, you can say that your role changed as it was felt that the secretarial role was more suitable for your skills.
If the interviewer needs to know more, they'll take a reference.
Generally speaking you are supposed to sell yourself with your CV. If you went to a green grocer for a pound of apples would you expect them to have labelled them, "mostly good, but some bruises" ? Try to remain honest but push the highlights. I guess the percentage time you spent at each role has a bearing, but if you were case worker for a large proportion of it then personally I'd forget the demotion on the CV, but be willing to admit it if asked at the interview.