It seems that the CV itself gripped the inside of the drive flange instead of mating to the inside of the bearing. The washer solves all, it's only 2mm thick, I doesn't shorten too much and as you know Mr Ghost I'm not lower than the queen's nips so I'm hoping it wont bring any issues.
You're right that when machining for larger bearings they shorten the flange a little, to prevent the above from happening.
As for the diff, I love what I've experienced so far, I can see how Iain is 5 seconds a lap quicker round Cadwell with his diff, once I learn the limits I think it's gonna really be fun to drive.