As said copyright exists from the moment you write the song and the self addressed envelope is only to act as proof of first ownership - and yes it does stand up in court.

Also as mentioned - the letter should be registered and the registration sticker is best applied over the opening of the envelope (this may upset the chap at the post office - but it was what we were recommended to do in the 60's and it paid off!)we then lodged the envelopes with our solicitor (or you could use a bank box etc) but of course you could keep it locked away yourself (don't loose it though!!!!)

Originally I was worried that once the envelope was opened in the court I would loose my proof (I was young!!) but of course the court's ruling and acceptance of your proof of date goes on record and if anyone else claims rights they would need to have proof predating the courts ruling

Not much added to the threads already posted but another angle from someone with first hand knowledge!

Pete