shower?

I am trying to replace the shower head holder on my trevi therm mk2 shower but i have been unable to figure out how to remove the bar which holds the shower head holder so i can slip the old holder off and replace it with the new shower hear holder. There was a small screw head type fitting inside the bar which i have removed however there seems to be some kind of fitting beneath that but i am unable to see how to loosen it or whether it requires a special tool to remove it. Any help would be greatly appreciated.