Fill with water the bottom half of a steamer large enough to hold a 1litre (1pint 15 fl oz) pudding basin and bring to simmering point. Brush the basin with oil, then spoon in the syrup.
Finely grate the zest from 1 orange, then squeeze out the juice and set both aside. Peel the other orange, slice it thinly and arrange the slices inside the basin.
Stir together the breadcrumbs, flour, suet, sugar and zest. Mash the banana and add it to the mixture, then beat in the milk and enough orange juice to form a fairly stiff mixture.
Spoon the mixture into the basin, without disturbing the orange slices, and level the surface.
Cover the pudding with baking paper and a double thickness of foil, pleated in the centre. Tie them to the basin with string and make a string handle to lift the hot pudding. Place the basin in the top of the steamer, cover and cook for 1hour 30 minutes. Top up the pan with boiling water during steaming if necessary.
Remove the basin and take off the paper and foil. Run a round-bladed knife between the pudding and the basin, turn out the pudding onto a plate and serve with the custard.