@MyAmpleSheep
You are (understandably) confusing two different things.
When looking at breach of contract or negligence, the question is whether RCN's Legal Services department was negligent.
Misunderstanding (or being ignorant of) the law is not negligence. Negligence means failing to exercise the care that a reasonably prudent person would use in a similar situation, resulting in harm to someone else. So they are entitled to argue that, given that their understanding of the law was a common one that was shared by the courts at the time, a reasonably prudent person would have acted as they did.
I know this can be difficult to get your head around, but it is different to, say, an employer being taken to court for failing to provide single sex toilets. That is not a question of negligence. That is a straightforward question of whether they have broken the law. In that case, misunderstanding or being ignorant of the law is no excuse. If an employer failed to provide single sex toilets and make sure they were genuinely single sex, they have committed a criminal offence.
However, when you argue that someone has been negligent, there has to be more than them simply misunderstanding the law. If that were not so, every single lawyer who gave advice to clients based on the decisions of the lower courts in FWS before it got to the Supreme Court could be sued for negligence, as could every single lawyer who advised the Scottish Ministers that they could win their case. In reality, any attempt to sue on this basis would fail. These lawyers were exercising reasonable care in giving their advice, so they were not negligent even though it turned out they were wrong.