Thanks to a mention by Ed Mitchell at com-prac I’ve come across an old (2006 :-)) piece at Dave Pollard’s blog.
Beyond the details and comments (which are most valuable), the logic for each step of the decision tree is quite solid. So if you’re having to explain (or make) tool and channel choices, or draw communications policies, it may come in most handy.
The following is Dave’s flowchart. But don’t miss his text.