DC schemes fours times worse than public sector pensions

That's the way to respond to this kind of thing. The PPI report is worth a read if you're a pensions geek, but the TUC is right to warn that its findings will be selectively quoted. And below is exactly how we should argue these points (in Lakoff-speak, don't accept the frame):

'The problem is not fair and proper public sector pensions, but the retreat by private sector employers from their responsibilities. The solution is to level-up, not strip away pensions from vital public servants.'

The most important questions are why are private sector DC schemes so bad, and what do we do about it. Everyone sensible in the pensions industry acknowledges that the contribution rates to DC schemes are far too low to pay out pensions anything like as good as the DB schemes they replaced. So let's fix the problem rather than trying to make all pension schemes equally bad.