Thanks for your response. From your screenshot it looks like your are using a different version to me (I'm experiencing this behaviour in v5.0.4). However, I have tried your suggestion and it does work as you suggest on apex.oracle.com (so v18.2) so it appears this could be something specific to v5.0.4. Ironically hiding the dialog close 'X' in v18.2 isn't possible using the 'usual' approach taken with v5.0.4 so I can't 'fully test' what I was hoping to achieve; although hiding the 'X' isn't recommended by some anyway so I'm re-evaluating whether there is a 'better' way.
From what I've seen of APEX 5.1 and up so far, it seems a version upgrade would be worthwhile...