TO MAKE THE CREPES: Sift the flour, salt and sugar into a bowl and make a well in the center. Mix the eggs and milk together with the water and pour slowly into the well, whisking all the time to incorporate the flour until you have a smooth batter. Stir in the melted butter. Cover & refrigerate for 20 minutes.
Heat a crepe pan or a non-stick frying pan and grease with a little butter or oil. Pour in enough batter to coat the base of the pan with a thin even layer and tip out any excess. Cook over moderate heat for about a minute, or until the crepe starts to come away from the side of the pan. Turn the crepe and cook on the other side for 1 minute, or until lightly golden. Stack the crepes on a plate with pieces of wax paper between them and cover with foil or plastic wrap while you cook the rest of the batter to make 6 large crepes.
TO MAKE THE FILLING: Preheat the oven to 350°F. Heat the 1 oz butter in a frying pan, add the mushrooms, season well and cook, stirring, for 5 minutes, or until all the liquid from the mushrooms has evaporated. Stir in the cream, cheese, and ham.
TO ASSEMBLE: Lay one crepe on a board or work surface. Top with about a sixth of the filling and fold the crepe into quarters. Place it on a baking sheet, and then fill and fold the remaining crepes. Bake for 5 minutes and then serve immediately.