You can get these nodes using
/Log/Override/DocType | /Log/Override/Purpose
I guess you're trying to say that your document might be different from this one. You need to explain how different it might be, and what criteria you are using to decide which nodes you want to select.
(Really, I despair at questions like this one. Do try and read your question after you've written it, and ask yourself whether it's likely to make any sense at all to someone who can't mind-read).
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference