Preheat oven to 180C/350F. Grease a 20cm (8 inch) (base measurement) springform pan.
Beat butter, rind and sugar in a small bowl with an electric mixer until light and fluffy. Beat in eggs, one at a time, until combined. Transfer mixture to a large bowl. In two batches, stir in sifted flour and baking powder then milk. Spread the mixture into the pan.
Peel apples; cut into quarters and remove cores. Make lengthways cuts into the rounded sides of apple quarters, cutting about three-quarters of the way through. Arrange quarters, rounded-side up, around edge of cake mixture.
Bake cake for 1 hour or until a skewer inserted into the centre comes out clean.
Sprinkle gelatin over the water in a small heatproof jug. Stand jug in a small saucepan of simmering water, stirring until the gelatin dissolves. Stir in jam.
Spread half the jam mixture over the hot cake; cool cake in pan. Remove cake from pan and brush with remaining jam mixture.