have seen a few helmets and the sizes of some large are advertised
by other manufactures as x large
, my big head won't fit a large and some x large are not much differant
my big bonce is a 61cm so would a 65 be ok,or how much leeway
should i have

 

i would recommend you try it on. dont mail order with out trying one on mate, not worth it!
As you say all makes are different.