Yeah i still have the original CA ;)
I found the solution anyway, was a strange problem, heres a post I left on another forum:
When looking at the ca.crt file, I noticed that after "State or Province Name (full name) [LONDON]:" there was "\x09":
Subject: C=UK, ST=LONDON\x09, L=LONDON, O=OpenVPN
This was seen as just some spaces when trying to build a key, however \x09 is actually hex for the tab key, I must have pressed it by mistake when first creating the ca file, so basically now when building client keys, i just type LONDON then hit tab, then enter.
What I dont understand is how I managed to create the first 16 keys without pressing the tab key!
Anyway, thanks