Global Header

I have a number of Webservice that require a login certificate to be embedded in the header of each request as a security measure, I do this by adding a new header to the request (button 'Headers' at bottom of page). Currently if I have 5 requests I want to run I have to manually add the login certificate to all 5 requests.

Is there any way to set a global Header for all requests of a project so if I generate a new login certificate I can easily share it between all my requests?

(Hope that makes sense).