I am trying to locate the music that was especially written for
Princess Margaret's wedding to Lord Snowden.  The Wedding March was
written so that when she started her walk down the aisle of
Westminster Abbey, it ended when she reached the altar...about 700
feet.  Can you tell me where to find either a recording of it or the
sheet music.  I think it was written by Sir Arthur Bliss.

I have found no references to a wedding march composed for the occasion.  

Two pieces that were commissioned for the wedding were "A Fanfare for
the Bride" by Sir Arthur Bliss and "Processional for the Wedding of
Princess Margaret," by W. Harris.  The latter was apparently written
for the Queen's processional in connection with the wedding, however.