Put the duck breasts in a shallow roasting tray, skin side up, and roast in a preheated oven, 200°C (400°F), Gas 6, for 10–15 minutes – they should still be pink in the centre. Leave to rest for 5 minutes.
Meanwhile, put the lentils in a saucepan of salted water, bring to the boil and boil for 15 minutes, then drain.
While the duck and lentils are cooking, make the marmalade. Put the orange juice and mandarins in a stainless-steel saucepan. Bring to the boil, then reduce the heat and cook for 10 minutes, or until reduced by two-thirds. At the same time, for the sauce, heat a dry nonstick frying pan, add the shallots and cook gently for 2–3 minutes. Add the port and grape juice. Bring to the boil and boil for 10 minutes, or until reduced by half.
Remove the duck from the pan and set aside. Skim off the excess fat from the pan. Add the cooked lentils and stock and cook over a medium heat, scraping any residue from the pan, for 2–3 minutes until the stock has nearly all evaporated.
Slice the duck breasts. Divide the lentils between warmed plates and top with the duck slices. Spoon a little marmalade over the meat, pour the sauce around the lentils and serve garnished with chervil sprigs.