GetDlgItem ID of the "Next" button?

What's the GetDlgItem ID of the "Next" button on a custom page? Googling around (Eg http://forums.winamp.com/showthread.php?postid=1702648) suggests that it's "1". Reading the source suggests that it's "1001", but in both cases this code sets "$1" to "0" after the GetDlgItem call...

  ${if} "$SAPageIsVisible" == "0"

    !insertmacro MUI_INSTALLOPTIONS_WRITE "software_audit.ini" \

       "Settings" "NextButtonText" "$0"

  ${else}

    GetDlgItem $1 "$DialogHandle" 1001

    SendMessage $1 ${WM_SETTEXT} 0 "STR:$0"

  ${endif}

Any ideas?
Angus

Sorry to answer self, but I continued browsing and discovered that the main dialog has a different window ID to the custom page. This works:

    GetDlgItem $1 "$HWNDPARENT" 1

Angus