If I were you, I think the first port of call would be your Union.
Write, don't ring, that gives you a timeline & documentation if you need it.
I would have thought that any settlement would be retroactive; so put this in your letter as well.
As a general observation, I would think that the issue may not apply to those who retired before 2015 ( or whenever the discrepancy arose); I don't believe you would be excluded because you retired after the action started.
In any event bringing your own action, even if accepted by the WRC isn't likely to move any faster at this time.