Graham The _headtags file is included raw and unfiltered into all pages and cannot be disabled so it should only be a text file not a full html page and only contain code valid in headers _header and _footer are included as the first and last items on the page and they are filtered (only the content inside the body tags is used)
_header and _footer can be disabled from the page options
it is possible to disable or change the _headtags file using a xtscript in the _headtags that checks the page name before being outputting the code