Having been in a similar position to your children, my advice would be to reflect on what you are trying to achieve here.
Assuming you have been an otherwise good parent, you are focusing your energy and frustration in completely the wrong direction.
Regardless of what CAFCAS may or may not have written in their report, only your children know how they feel NOW, and whether that is derived from their own experiences of your parenting, or from being influenced by the resident parent.
If they were much younger children, there might be value in pursuing the matter, as re-establishing contact with younger children would give you the opportunity to build/re-build the relationship. However, given the ages you have said they are now, this is not the time to keep pushing this - teenagers are stubborn at the best of times, and I think it would have the opposite effect. Even trying to get a decision from the court on whether or not they have been alienated is not going to stand in your favour later on.
Children (even ones who are now adults) generally don’t appreciate a bit of paper saying non-resident parent was right. If there is a relationship to be salvaged/re-established, then what is appreciated is knowing that their choices at the time (however upsetting for the non-resident parent) were respected, that their non-resident parent continued to support them financially, and that there was always an open channel for the child to reach out if/when they were ready.
you've had good advice already about this, from more than one poster. my only other suggestion on that point is to consider other ways to communicate.
when I was an angry teenager, the birthday cards etc that arrived from my non-resident parent went straight in the bin without even being opened/read. I’ll never know now what was in them.
Maybe set up an email address or something that you can write to, send photos etc, and make sure they are aware of it - they might not be ready now, but when they are, they will be able to see how often you were thinking of them, and the messages will be there then.