The situation would be - you find your copyright abused, you advise your solicitor of your sealed proof, and it is opened in fron to witnesses, and if necessary, the case proceeds. You would need proof that your envelope was sealed at the time of posting - I'm oretty sure the PO do this - they used to run round the envelope in straight lines with blue felt tip, to prove it had not been tampered with, but you;d need to clarify that. You could try the 'sealed afterwards' method, but if the person you are trying to sue has done it the correct way. you're going to look rather silly!