How do you draw adverse inferences when you don’t know what something says?

There’s a key document (an investigation report) not disclosed in full in my case, and I’ve resigned to the fact that I won’t get it and I’ll have to ask the tribunal to draw adverse inferences from its omission. But if I don’t know what is contained in it, in what way can I ask an adverse inference to be drawn? Do I just ask for the best inference that suits my claim to be drawn? How about where there is evidence which does not support my case that has been disclosed - can I make the case that in the absence of the investigation report, the evidence which has been disclosed should be ignored/given little weight?

Thanks!