The Channel Islands consist of 2 States -
The Bailiwick of Guernsey which includes Sark, Alderney, Brecqhou, Jethou and Herm
The Bailiwick of Jersey
Their records are completely separate.
There is information on Sark here:
http://chi.genuki.weald.org.uk/srk/startIt states there:
Birth, Death and Marriage records from 1925 can be found at:
The Greffe, La Chasse Marette, Sark GY9 0SF.
Apply direct to the Church for 20th Century registers