Boiling point on the Exeter line

The air conditioning is woeful on the diesel class 159 trains serving Exeter from Waterloo. It's often broken and even when it is working, it's not powerful enough for a hot summer day.

The livery / interior has been upgraded. Why not the a/c?

Your poor sweating guards battle the heat by opening the TINY windows but their efforts are in vain. We are trapped in moving greenhouses where temperatures are regularly nudging 40 degrees. 

Looks like we're stuck with the class 159 for a few more years so what's the plan?