Tiramisu
Thanks for the compliment.
You can only sue officers of the Union, I think. Unions are effectively Trusts and as such have no legal personality. So you can only sue the officers. I believe you are not a member, so your scope to sue them is less than if you were, ie they probably owe you a lesser duty of care than their members. I say probably, because you are caught by the collective bargaining agreement even though not a member, which confuses things a bit.
As to the officers' liability, there is probably something in the Union legislation that gives them some protection. Otherwise nobody would ever volunteer to be a rep! I suspect legally reps are viewed more as "knowledgeable amateurs" rather than "experts". Full time officials are probably deemed to have a greater level of knowledge (even if they don't in practice). That's why TW joining the BASSA panto act is intriguing. He probably has a lot more to lose if members decided to challenge him over Unite's handling of the dispute. Going after someone like Holley is less likely to deliver a result, unless the claimant can show very obviously that the BASSA reps knowingly put their own personal interest above those of the wider membership.
In general, winning a tort action (eg negligence) is much harder than winning a breach of contract case, and I doubt there is a breach of contract case here against BASSA. If I'm right that you're a non-member, then you personally definitely have no case. Even members would struggle, though. I see little evidence that BASSA has failed to deliver the services it contracts to do so in return for membership fees (I'm assuming this is structured as a contract, which may well not be the case anyway). BASSA might have provided the services badly, but they have provided them. Anyway, legal action is overrated. It's expensive and takes time. By the time you've got a decision you've usually forgotten what the issue was.