The following gave me a bit of an ‘Aha!’ moment:

‘There is no form inherent in expository preaching. Richard Mayhue is on target when he writes, “Exposition is not so much defined by the form of the message as it is by the source and process through which the message was formed.  To put it another way, there is no such thing as an expository sermon form. Ideally, the form should come from the text. As Sidney Greidanus argues, an expository sermon should impart a Bible-shaped word in a Biblelike way.’

(Stephen D. Mathewson, The Art of Preaching Old Testament Narrative, p22)

If this is so (and I think it is) it means that the expository sermon and the narrative sermon should not be viewed as alternatives, but rather as potentially complementary aspects of the same sermon: expository in content, narrative in form.