Hi GlasGaz74.
I understand the point you're making and why you are making it.
It is mistaken though. The point isn't whether people knew about PPI claims at the time. The point is that PPI had been missold to you before that time. This (at the time) unknown potential for a claim was an asset that vested in your trustee.
You might have had a long-lost great-aunt in Australia who'd died just before your trust deed began. She'd unexpectedly left all of her assets and money to you. You didn't know her plans. You didn't know she'd died. There was no way you'd have been able to tell your trustee in advance, or that they would have had any reason to ask you about it. You found out about it a few months later when an Australian solicitor contacted you. This would still be an asset that vested in your trustee when your trust deed was signed.
It seems to me that RBS are going to have to either change their policy, or go back to Court to make their argument again. I agree (from a non-legal-specialist point of view) that their current position seems unsustainable.