How can I have a different header for different sites in a WPMU setup ?

Hi

I've got a WPMU site setup (my first!) and I have the same theme in the 'Master' as I do on the sites - when I edit the header.php file in any site it simply updates - is there a way of making specific changes to just one site ? ie the Master and for it not to show on the others or vice versa

Thanks

John