Greenstar CDi 37 showing low system pressure ?

My brand newly installed (5 months) Greenstar CDi 37 is showing low system pressure of less than 0.5 all the time. The rads make a real gurgling noise and when the timer kicks in first thing in the morning there is a loud gushing noise as the water fills into the rads upstairs. I am continually bleeding the rads and ther is some black stuff comes out 2 of the rads (which runs clear after a few seconds). How do I re-pressurise the system? What is the likely cause of this?