Employers quite often refuse to disclose documents that mention or relate to other individuals, saying they have to keep those documents confidential because of the Data Protection Act.
They are wrong. Section 35 of the Data Protection Act says this:
(1) Personal data are exempt from the non-disclosure provisions where the disclosure is required by or under any enactment, by any rule of law or by the order of a court.
(2) Personal data are exempt from the non-disclosure provisions where the disclosure is necessary—
(a) for the purpose of, or in connection with, any legal proceedings (including prospective legal proceedings), or
(b) for the purpose of obtaining legal advice,
or is otherwise necessary for the purposes of establishing, exercising or defending legal rights.
In other words – in this context – there is nothing in the DPA to prevent an employer from disclosing documents if disclosure is necessary in employment tribunal proceedings. At most, if the relevance of your request isn’t obvious, they might say that they are not sure that section 35 applies because they are not sure disclosure is ‘necessary’ for the stated purposes – so if you want it you’ll have to get the tribunal to order it. But that doesn’t mean the DPA will be of the slightest help to them in opposing your application: you’ll just be arguing about whether disclosure is necessary, which you’d always have to establish before you got an order for disclosure.