We purchased two Alpha 200 helmets from SES. The RAF helmets are too heavy and the field of view of an Alpha is better.
Cheers sharpend.
Interestingly it is, actually, illegal to purchase and wear ex-RAF helmets. When I rang the manufacturers to ask for a size chart I got a very curt idea where I should put it
p.s. Please don't listen to the naysayers. Carry on wearing that helmet.