worcester 28CDi,unable to pressurise?

My Worcester 28cdi had lost pressure, It was down below 1 bar,I filled it as per instruction book ,however I inadvertantly filled it to 3 bar,I then released the pressure via the pressure relief knob,I returned to the unit after 20 mins I noticed the pressure gauge was at zero. I tried filling the unit again but after a few mins of filling it the pressure relief valve started knocking and the water started flowing out the overflow. Is there is a blockage in this valve?,I assume this needs stripping or replacing ,I have no plumbing experience, it it a job that a lay man could attempt?