Preheat oven to 180C.
Peel, core and dice apples. Place apples, water and lemon peel in a non-stick saucepan. Cook over moderate heat until soft but still firm. Stir occasionally to prevent apples from sticking. Remove from heat, add sultanas, sweetener and cinnamon or nutmeg. Remove lemon rind.
Lay a sheet of filo pastry on a greased baking tray. Lightly brush pastry with some melted margarine. Place another sheet of pastry directly on top of first sheet and brush with margarine. Repeat the process with remaining two sheets.
Lay apple mixture along length of sheet, leaving at least 5cm from sides and edges. Fold the sides and edges over. Gently roll mixture up to resemble a Swiss roll. Brush edges and top of strudel with remaining margarine. Slash top diagonally with serated knife 3 or 4 times.
Bake in a moderate oven for 30-40 minutes, or until golden brown.
Serve hot or cold, topped with reduced-fat ricotta cheese.