Lots of employment tribunal cases are won or lost depending on how thorough a job the claimant has done of shaking all the relevant documents out of the respondent.
A particular point to keep in mind is just how much communication is now done by email. That can be easy to overlook if your own job doesn’t involve sitting at a computer all day. But for those who do (and that means almost all office-workers), many things that a few years ago would have been said on the phone or face-to-face are now put in an email.
The difference between picking up the phone or putting your head round the door and writing an email, of course, is that the email leaves a permanent record. So if your managers have been discussing by email how to go about sacking you (for example), that is material that you should be able to get hold of when it comes to disclosure.