While setting up Views RSS so that I can use Mailchimp on Poligo, I came across this good guide to using fields for RSS.

Points to be wary of:

• For the <link> element, use "content: path" and in field configuration section (Rewrite results fieldset) check Use absolute link (begins with "http://");
• The element <author> should contain the author’s email address. In D7 and D8, start with adding Content: Author relationship. Use field User: Email. In field configuration section: in Link this field select No link;
• <Category> should be Content: All taxonomy terms. The manual recommends “in the field configuration section D7, in Display type select RSS <category> element“, but this was not available.
• For <GUID> use path.

UPDATE: I found a much easier way to acheive what I want. There is an RSS display for each content type. Creating an RSS feed view of content, and then choosing what I wanted in admin/structure/types/manage/blog/display/rss did the business. The ony issue now is image size in the RSS reader.