Preheat oven to 180 C / Gas 4. Grease and flour a 20x30cm or similar sized tin. Sieve together 250g flour, bicarbonate of soda, cinnamon and salt. Set aside.
In a large bowl, cream 400g sugar and 225g margarine until light and fluffy. Beat in the eggs one at a time, then stir in 2 1/2 teaspoons vanilla and 2 tablespoons water. Beat in the flour mixture, stirring just until incorporated. Finally, stir in chopped apples and pecans.
Pour batter into prepared tin and bake in preheated oven for 40 minutes or until a skewer inserted into the centre of the cake comes out clean. Allow to cool.
Meanwhile, make the rum sauce. In a small saucepan, combine 250ml water and 110g margarine. Bring to the boil. Mix 200g sugar, 2 teaspoons flour and 1/8 teaspoon salt and stir into the saucepan. Bring sauce to the boil, stir in 2 1/4 teaspoons vanilla and 2 teaspoons rum essence. Pour sauce over the cake and serve.