Where are you working MichaelBratton? I am from Australia so can only speak from my experience here.
Where have you looped your circuit, behind the switches or in the ceiling (ceiling roses or junction boxes)? Is it possible that you may have crossed a switched wire and a neutral? Have you overtightened and subsequently cracked a socket?
Have you completely rewired the house or have you used all or some of the old accessories? Have you pulled the cables in while removing the old cables (i.e., used the old wiring to pull the new cabling into place). Was the old installation in metal conduits...maybe rubber and cotton wound insulation (very old)?
If the circuitbreaker seems to be popping when you close it, it sounds as though it is closing onto a fault, but a resistive one, since it shouldn't close onto a fault at all (trip free mechanism!). It doesn't sound like an earth fault since the RCD isn't tripping now. Maybe tightening the loose neutral earlier bought this fault to light downstream.
Is the house occupied? Can you turn off the supply. What insulation resistance should you be getting...what reading are you in fact getting?
I am not just answering your questions with my questions, just trying to get you thinking along those lines. These are issues I have encountered in my work and a few of the causes.
Let us know how you go.
Best regards Phil