Season the duck breast skin. Heat the oil in a frying pan over a moderate heat and add the duck breasts, skin-side down. Cover and cook for 3-4 minutes, until just lightly browned. Turn the breasts over, lower the heat slightly and cook uncovered for 5-6 minutes.

Peel the skin and pith from the oranges. Working over a bowl to catch any juice, slice either side of the membranes to release the orange segments, then set aside with the juice.

Remove the duck breasts from the pan with a slotted spoon, drain on kitchen paper and keep warm in the oven while making the sauce.

Drain off the fat from the frying pan. Add the segmented oranges, all but 30ml/2 tbsp of the orange juice, the port and the orange marmalade.

Bring to the boil  and then reduce the heat slightly. Whisk small knobs of the butter into the sauce and season with salt and pepper.

Blend the corn flour with the reserved orange juice, pour into the pan and stir until slightly thickened. Add the duck breasts and cook gently for about 3 minutes.

To serve, arrange the sliced breasts on plate with the sauce.