Thanks for all the comments all.

At the end I did a lot of reading of reviews, tried on a ton of helmets and ended up ordering in a Shark RSR2.

It fitted me very well - snug all over. What came as a real surprise me was the HUGE difference in shapes between brands.

A med in shark was great. A soumi med only came 1/2 way down my head and a med Nolan fitted great except for the fact my chin couldn't fit in the helmet.