Heat oven to 160C/140C fan/gas 4. Toast the star anise and coriander seeds in a dry pan until aromatic. Tip the toasted spices into a spice grinder with 2 tsp sea salt and grind into a fine powder (or crush using a pestle and mortar). Put the spice salt in a bowl, add the sugar, mix well and set aside.
Lightly score the skin of the duck in a criss-cross pattern and heat the oil in a large casserole. Using a pair of tongs to turn it, brown the duck well on all sides, pour off the excess fat, then sit the duck breast-side up and season all over with the sugar and spice mix. Pack the plums around the outside, then scatter over the bay and pour over the vinegar and stock.
Roast in the oven for 2 hrs or until the duck is golden and the plums have broken down. Turn the oven right up for 10 more mins to crisp up the skin. Take the duck out of the pan to rest for 10 mins and spoon the excess fat off the plums. Carve the duck and serve with a good spoonful of plums.