Thanks for the help guys. A collector I knew from my days in the Navy got in touch, and clued me in. The helmet actually DOES fit, it's just down to my being an 'odd head' (elongated shape. size 58 by width, size 61 by length.) So he helped out with tips on heating, and after about half an hour, and the 'necessary evil' of veg oil on the temples...the helmet slid on like a glove, and fits BEAUTIFULLY once on, with plenty of space, and no squashed body parts (even wobbles a little). Thanks guys!
(Progress shot - about 40% of the way in...please be gentle :-D )
(Don't know why pic is on it's side)