You could think of it like they're all doing exactly the same job, and then a union voted to strike, as a result of which only the unionised workers got a bonus

You could think of it like they're all doing exactly the same job, and then a union voted to strike, as a result of which only the unionised workers got a bonus, and were kept at the same grade. (Although I'm not really sure what my point is - that the council's lawyers should've (if they didn't) made that argument? - because unlike gender I don't think union membership is a protected characteristic per Equality Act 2010, obviously there are some different rules like you can't fire someone for being a member of a union, but paying someone less because they're not a member of a union seems probably fine? But anyway, as a public body the council has extra requirements for equality, not less, and the intention probably doesn't matter at all.)