Some actual information:
http://www.snopes.com/legal/postmark.asp
Quote:
However, the U.K. Patent Office has this to say on the subject in its "Claiming and Enforcing Copyright" FAQ:
How can I prove originality in my work?
Ultimately this is a matter for the courts to decide. However, it may help copyright owners to deposit a copy of their work with a bank or solicitor or send a copy of their work to themselves by special delivery (which gives a clear date stamp on the envelope), leaving the envelope unopened on its return; this could establish that the work existed at this time. (Further details of special delivery should be available at Post Offices).
Careful readers will have noted the "may help" and "could establish" in the above entry.
So, basically, it is admissable as evidence but far from conclusive.
Sending it to a solicitor or depositing in a safety deposit box is
better.
Ultimately the best protection is registering the copyright with PRS / MCPS or whatever it is.
This will cost you though - but then again, as they say oop north, you don't get owt for nowt.