I have created an adm file to do this through group policy, just copy and paste the below into notepad and save as a .adm
CLASS MACHINE
CATEGORY "Sophos"
POLICY "Disable Sophos Expiry Msg"
KEYNAME "SOFTWARE\Sophos\SAVService\Application"
EXPLAIN "This policy disables the expiration of sophos 7 messages"
VALUENAME "DisableEndOfLifeMessaging"
VALUEON NUMERIC 1
VALUEOFF NUMERIC 0
END POLICY
END CATEGORY