high pressure and leak?

my ferroli modena 80e is leaking inside from pipes and outside through pipe. the pressure is at 3 bar, tried reducing it with valve, and it does until i close the valve then it moves back up to 3. the noise is constant but when i turn the valve it stops so obvously its the pressure causing the noise. just looking over this forum has confused me.had a man in who said it was the safety valve, but now im thinking is it the expansion vessel or the heat exchange...the man took 2 mins to diagnose and said it would £165 for parts an labour......just unsure. help please !