That article is a bit scant on details. In particular this part:
The vote for industrial action follows the union's rejection of an offer of a pay rise between 2.7 - 4.6 percent.
Every article I've seen is for the offers to be over 3 years or so. This article makes it sound like they are turning down an offer of three or four percent which seems like a reasonable offer at the moment, is this amount actually split over several years making it far less than inflation like the previous offers?