The Union reps do this voluntarily, much of it in their own time for no reward (no, we are not paid for it)
Indeed, we're not paid, we do it for the greater good.
As an aside, I read a section of the PCS mag at work this afternoon and it said much the same thing about negotiating pay deals (as in the Prospect mag) and cited rises in food, fuel, council tax etc as a reason why workers should not be forced to accept poor pay deals..............except for their members at NATS obviously.