[kiwidude]

Well you've got a few choices. You don't "have" to have a class, you can just declare normalise at the module level (reduce the indentation) and then change your show_dialog so it just says:

    def show_dialog(self):
        from calibre_plugins.ag_normepub.main import normalise
        normalise()

def normalise():

Or if you still want to have a class, change PlugMain so it doesn't inherit from QDialog:

    def show_dialog(self):
        plug = PlugMain()
        plug.normalise()

class PlugMain(object):
    def normalise(self):

I'm assuming in the above you don't need any of the gui related parameters, if you do just have a briefer constructor.