With regard to proving/knowing who was effected, I'm pretty sure they can find out exactly who was effected. I just depends if its easier for them to just dish out a discount to all vs spending the effort working out who was effected. Not hard for them to do it, but will require a bit of effort and might not be cost effective. No way you can easily prove you were effected and that would take a lot of man power for them to process all the claims. Cheaper to just give the whole base a discount.
From the BBC Article.