To be fair to the unions (and I am not fair to them very often), they only want what their employer has already agreed to in the past. We plan for our retirement based upon what we expect to have from our occupational pensions, and if this changes for the worse towards the end of our careers then we could be facing an old age of poverty.
From my point of view, Gordon Brown was stupid enough to arrange my PAS pension to be quite lucrative and having retired I am living to an agreeable standard without having to have taken out additional pension savings, or having to seek additional employment. I would not wish a sudden fall in expectation at a late point in a career on anyone. However, if new pension arrangements only affect future recruits then that is fair. (I do not want to get into an argument about the future military pension changes - I am not affected -yet).