Crepes: Sieve flour and salt into a bowl. Mix the egg, milk and oil well in another bowl. Add the liquid ingredients to the flour and salt mixture. Leave the batter to stand for at least 30 minutes.
Heat a small piece of butter in a frypan over a medium heat. Add a small ladle-full of the batter. Swirl the frypan straight away so that the batter is spread evenly across the bottom. When the bottom side is lightly browned flip the crepe and heat the other side until lightly browned too. Do the same with the remaining batter.
Sauce: Melt the butter in a frying pan. Add the sugar, orange zest and orange juice then bring it to the boil.
Serving: Fold each crepe in half twice so they're in quarters. Dip each crepe in the hot sauce and place on a warm plate. Add the Grand Marnier to the same pan and heat up and pour immediately over the crepes.
Place them a little bit away from you and guests (and for extra effect turn the light off above the dining table) and use a long match to flambé the crepes. Serve immediately.