The "normal" method of overriding the default text on controls is explained in the documentation and is to use {{ form_label( form.thing, 'New text') }}. This doesn't make any sense (and is completely ignored) for submit type controls.

The actual method is to do {{ form_widget( form.save, 'Press this!' ) }}. However, that isn't specifically mentioned as far as I can see. A couple of lines about this would have saved me a bit of time!