I was a Vulcan AEO in the late 70s and I replaced my inner and outer helmets for a one-piece job - a Mk4a I think. The old inner helmet made my head itch and I was far from convinced that if a low level abandonment was called for I would have time to don the outer helmet before exiting the aircraft. The Mk 4a seemed to be a bit heavy at first but I soon got used to it and I definitely preferred it to the two-piece head gear that I had earlier.