After digging the C2 out of the back of the wardrobe to take it to Infinity, I had a play to see why the lock on the left wasn't closing properly. I noticed that the lock that goes over the locking posts at each side is in effect a hook.
On the right side the hook is in one place, and on the left it is further away, sort of leaning back, so that when the chin piece is pulled down it clashes with the head of the locking post. I moved it back with a fingernail and it was fixed. Sadly, the fix is only temporary as when you open the chin piece it flicks back into the wrong position, so something is letting it move out of line.
I gave it a run out to work today. At first I didn't notice the increased quiet in the helmet but overall it is a quieter helmet than my other choices, both Nitro's. I always wonder if other C2's have the same floppy visor? With the Nitro I am used to a stiff action that keeps the visor down, but the C2 is much easier to move up and down; far less effort required.
It's hard to describe the fit of the helmet but it is a better all round fit than anything I have owned sine the Shoei GRV on the 1980's!! It seems to fit all my head and neck, the flaps under the ears and in the chin piece make it feel closer fitting than other helmets.
Another plus point is the flip down sunshade. No longer original but a useful tool when it is very bright sunshine.
I think it still needs to go back to Infinity but at least I know what the problem is.