Taxonomy and the default header

I've been playing around with Header image and I've managed to get each of the sections of my site to show a different header using taxonomy and I works well.

How though do I then set a default header for the front page and all the other pages like the administration ones?

I tried adding the php option and entering the return true and setting the weight to 10 mentioned in the documentation but that just puts the default header on every page - overriding the taxonomy settings. I've tried fiddling around with the weight settings but can't seem to get it to work.

I'm probably missing something obvious but a few pointers would be really appreciated.