Thanks to everyone that recommended I look more closely at XPath for doing this - I wanted to show the simplest approach (which to my mind is always SimpleXML) but I've adapted my own code now to use XPath. I do love it when I learn something from my own blog :)

Mario: Thanks so much for this explanation! The URIs won't change but I should register them explicitly ... that feels more comfortable than relying on the abbreviation as I did in my original example.