Ah ha!
I've just worked it out.
The answer is to use View Composers
(one of the Laravel concepts I never properly investigated..)
So in my routes.php (temporary placement - I should probably place composers in their own auto-loaded file) I've just added the following.
View::composer(array('layout.master'), function($view)
{
Asset::container('header')->add('bootstrap-css', 'bundles/bootstrapper/css/bootstrap.min.css');
Asset::container('header')->add('main-css', 'css/style.css');
Asset::container('footer')->add('bootbox', 'js/vendor/bootbox.min.js');
});
Given that every frontend file I have is using /views/layout/master.blade.php then I know that I can apply assets to it by passing 'layout.master' to View::composer.
Et voila - now my 404 view is also loading the correct assets.