Grease the inside of a 1.2 litre (2 pint) pudding basin lightly and line the base with a small circle of nonstick baking paper. Spoon in the syrup, then thickly slice 1 apple and arrange in an even layer on top. Coarsely grate the remaining apple.
Cream the butter and sugar in a bowl until pale and creamy. Gradually mix in alternate spoonfuls of beaten egg and flour until both have all been added and the mixture is smooth.
Stir in the grated apple, orange rind and juice, then spoon into the pudding basin. Level the surface and cover with a piece of pleated nonstick baking paper and foil. Tie in place with string, adding a string handle.
Lower the basin into the top of a steamer set over a saucepan of simmering water, cover with a lid and steam for 2 hours until the pudding is well risen and a knife comes out cleanly when inserted into the centre of the sponge.
Remove the foil and paper, loosen the edge of the pudding and turn out on to a plate with a rim. Serve immediately with custard or ice cream.