Wozzer:
For anybody pondering the CPC qualification for any reason, I found this useful site full of stuff:
Hi Wozzer, Since a fair number of people take the CPC to become an O/D, there’s a sticky near the top of the O/D’s forum with links to the Nat and Int CPCs. 
Another reason that people might need a CPC is for career progression in a transport office, as some firms ask that supervisors hold a CPC.
Whenever I see a post asking about Nat CPC, I always link the answer to the sticky and advise that the guys follow the link and read as much as possible. 
It helps to know where the goalposts are, but then there’s the thing about horses and water…