I redesigned the head slightly making it wider and closer to the measurements I had originally.
Since I printed the test pieces in black pla to see how the carbon fibre tube fitted I decided to the 6mm tube was not going to be enough to prevent the neck from bending under the strain from the strings.
So, I've updated the files so the neck takes 8mm carbon fibre tube. This is cutting it close and doesn't leave much around it but should work out ok.
I still have the old files but think the new 8mm carbon rod will be better. I guess I'll be reprinting the test neck again.
I've ordered some 8mm carbon fibre tube from HobbyKing (The bloody postage costs more than the tube lol)