Put the soy sauce, sake, mirin, oyster sauce (or teriyaki sauce) and sugar in a small pan. Bring to a simmer to dissolve the sugar.
Preheat the oven to 140C/300F/Gas 2.
Place the pork into a small deep roasting tin. Pour over the hot liquid. Add the leek (or onion), garlic and ginger. Depending on the width of your tin, add 400-500ml/14-18fl oz of hot water – enough not to completely cover it as it will be turned every so often.
Lightly cover with a double layer of foil and leave a small gap for the steam to escape.
Turn the pork occasionally and cook until the pork is tender – about 2-2½ hours.
Once cooked, remove from the oven, leave to cool in the liquid and put into a sealed container with the juices. Leave to cool before putting in the fridge - it is easier to carve when it is fridge cold. If using straightaway leave to rest for 20 minutes.