I am a Union Rep, and would not give you information I believe would constitute "Work to Rule" , this would be withdrawing services like OT , weekend working , emergency callout etc..
I am only saying this is a usual trick, your direct boss tellin, ya you are great , and he would love to help but .... , and HR playin bad cop.
Do not take on extra responsibility, unless you can fit this into your normal working week.
Do not feel guilty about this, they made a commitment, and they have not met their side of the deal.
Explain this situation to your rep, ( I would phone if I was you) and if you are not confident enough to approach HR, ask him / her to do it.