Hi @JonMaguire,
In your previous question the answer was to use ${item.properties.subtitle} because in that case, the item in question was a Page object.
In the case of the Navigation component, the item is not a Page object but a NavigationItem object (you can see this by checking out the getItems() method of the Navigation interface) which does not have a getProperties() method, so ${item.properties.subtitle} will not work 😕 In HTL, the dot accessor is actually shorthand for using a Java getter, so foo.bar is the same as as writing (in Java) foo.getBar().
Instead, I recommend you make your own class that extends the NavigationItem interface and provides a getSubtitle() method.