breach of compromise agreement by employer.

Liability is determined not by what they "doubt" but by what it actually says - and it did not say that they would disclose no further information. I think that you would find that there is a vast difference between what they are prepared to add to the CA now, and what a court would rule about the actual wording, and what their lawyers would argue if you attempted to take this to court. A cosy meeting of the employer and the union is not an adversarial contest between lawyers in court. Courts cannot decide that it should have said something different than it did - your lawyer / union should have done a better job of it. It wasn't rocket science - this would have been a standard clause and its non-existence ought to have been questioned.

But in your shoes I would be far more worried about the former employee, and how they knew enough about you to be able to contact the potential employer in the first place, because what they have done once they can do again, and the employer has no control over former employees. It would appear that you have some enemies, and regardless of what the amended CA says, if this happens again all that will happen is the former employer will refuse to diclose any further details - it will not vindicate you of the allegations of this former employee and it will not help if the employer again asks you the question directly and you lie about it, and they already have this information from another source.

And I disagree that the actions of the former employer were the cause of the withdrawal of the offer. The potential new employer was clear - "I asked you specifically in our meeting yesterday whether there had been any disciplinary matters or allegations against you and you denied this." Whether or not the former employer had disclosed the information, they already knew that there were such allegations. The correct, and truthful response is "I am bound by the terms of a legal agreement not to discuss why I left my former employer and the circumstances leading up to the termination of my contract". Having a compromise agreement is not a licence to lie.

If the the new employer had contacted the old employer about the displinary and just been told "we are not in a position to confirm or deny" the offer would most likely have been withdrawn anyway. So any compensation is likely to be minimal.