In the present system if the CSS isn't able to be included within the page head (because it is to late to do so) then you get a warning within the block.
The problem is that the original way in which I had intended for users to allow the block to add the CSS to the head is no longer practical due to recent refactoring.
As a note to myself it should be possible to add reference to the CSS file within the block by overriding the specialization method, the moodle_page object looks as though it is available at that point.

Cheers
Sam