Not sure if the seat is the same as the 560SL, but check this thread, about 3/4 way down, I posted a picture of a simple homemade spring tool I made, from the service manual instructions, with some adaptations for the material on hand. My 2 fingernails are just now becoming normal again after getting them mashed by the return spring rachet mechanism, last May. Be careful in there!
http://www.peachparts.com/shopforum/showthread.php?t=222141&highlight=seat