You need to start preparing this dish at least the night before you want to cook.
Wash the duck thoroughly with cold water, drain and pat dry with kitchen paper. Leave to one side in a cool dry place for an hour or two if possible.
Mix together the sugar, star anise, ginger, spring onions and a few good pinches of salt, then use this to fill the cavities. Close the cavities with skewers and set aside.
Mix golden syrup with a tablespoon of the vinegar in a small bowl.
Add the remaining vinegar to a jug of boiling water and pour over the ducks. The original recipe suggests that boiling water opens up the pores of the duck skin, while the vinegar helps to strip some of the waxiness from the skin. Certainly, the end result bears out the suggestion that this will make it more receptive to the syrup, adding sweetness and a deep golden brown 'varnish' to the final dish.
Baste the duck well with the syrup and vinegar mixture, then place in a large plastic container and put in the fridge or a cool larder overnight, ideally uncovered.
To cook, heat oven to 220C/200C fan/ gas 7. Put a little water in the bottom of a large roasting tin, place the duck on a rack over the top and cover with foil. Roast for about 45 mins.
Take off the foil and roast for another 45 mins so that the duck is well cooked. Take the duck out of the oven, cover it with a foil tent and let it rest for at least 20 mins before serving