It seems it IS legal and the recordings will be admissible evidence in restricted circumstances - the employer must have genuine reason to believe that criminal or very serious breaches are occurring, and only for a period necessary to get the evidence.
I have had some major problems with staffs myself over the years and used cctv footage for "evidence".
I have found out only last year through police and the nfrn that you could not record staff without letting them know.
so check this one and be careful because the law is forever changing for employees.
I know that you can not use a "secret" cctv on young people
as it is called [ intraptment].
Sorry Ethel i am not saying that you are wrong so missmore double check .