One thing I have just discovered today is that the CX counter outside the barriers at the Airport Express check-in (open 7am to 10pm) is one place where you can show your credit card to validate any e-ticket bought anywhere on CX; once you have done this the person for whom you have bought the ticket will be able to check in anywhere on the CX network without problem. I wish I had known this sooner - I assumed a trip to the Peninsula Tower (with limited opening hours) would have been necessary.