I am intrigued to note that they didn't even do you the courtesy of providing a copy of the first assessment to take home with you. Was it a thing where they let you read it but then took it back and put it on your file?
The employment tribunal might not read any significance into that and I can see why you, yourself, didn't see the need to ask for a copy: At the time, you had no reason to believe that there would be a second round of redundancies.
I can -sort of- see the employer's point of view as well. The first round selects out people on performance grounds and even Union membership cannot save them from that. The second round could be about the state of the company's finances and "even adequate and good performers have to go" so the selection criteria are -genuinely- different from the first round. They won't want to admit it but, for all we know, non-Union staff and the people with service short enough to make the rendundancy payout affordable will be the ones to go.
Sorry that this doesn't actually answer your question but there are 'rights' and there are "right thing to do"s. You've set down a marker, as if to say "I'm going to make you do this properly", which is admirable but maybe what they really want is compliant employees who don't ask awkward questions or cause them 'trouble'.