Radiators?

Hi

I have a Ravenheat boiler for my central heating system. The pilot recently went out so the plumber advised I raise the pressure back to 2 BAR, swithc it off and back on again. This got the boiler back going but now all my radiators are making a clunking noise when they come on and off. I have bled them all so there is no loose air in the system. I think it may be the pressure- the red line is set at 1 so I think I have too much pressure in the system. How do you release pressure so the boiler is set back at 1 BAR?