Good work Dave, needed said. I read a thing years ago written by a Shoei race technician demonstrating the importance of fit and how force is transfered across a gap, it was roughly:
"Stand up straight, with your lower arm perpendicular to your body. Make a fist and walk to a wall and once your fist is against the wall, punch as hard as you can. Now take a small step back and repeat. That's why you need a helmet to fit."
IMHO the plastic adjusters at the back of helmets are only there to keep costs down, you can buy a basic motorcycle helmet in 5 sizes, yet most cycle helmets are only available in a couple of sizes. Top end motorcycle helmets use multiple shell and lining sizes, cheap ones use one shell and 5+ linings on one shell, for the same sort of money as a mid market cycle helmet.