That makes it a bit difficult to create a child language pack because you can't see the parent strings, so there's no easy way to know which ones might need changing.
I created a patch that seems to do what I want - it shows the parent strings when they are available and falls back to english when there is no string in the parent language.The patch is for moodle 1.9, but I've attached it here in case anyone else has a similar problem.
Simon