Well as treasurer of my club I pay 'out of pocket' expenses - they give me a receipt and I repay them what they have expended (out of their own pocket) on the club's behalf.
That is not the same as a flat rate amount. A flat amount, especially if it is to recompense for reduced income while you are rostered off duty, would be taxable, somewhere, and I can't see why HMRC haven't enforced this with BASSA reps, or any ex T&GW members, if they have already done so with CC89/Amicus. It wouldn't take years. A 'back years' investigation may take time, but not enforcing correct compliance from a given date.
Maybe they have, but you are not privy to any evidence to that effect. And if you are not a member of that union, you should not be privy to that evidence.
All you can do is persuade someone who is to find out. Or what Litebulbs said.