A hint from liliaeth and some more googling found me the bande dessinée I was trying to identify: it is Autonomes, the first of a trilogy called Chroniques de fin de siècle, by Jan Bucquoy (better known as a film director, including of La Vie sexuelle des Belges 1950-78). There is a plot summary here; apparently the eeevil French invade the freedom-loving anarchist Walloons, and there are two more volumes after the one I read (which was published in 1985). Now that I've identified it I may try and track it down via the local comics shop.