Cut the cabbage into large chunks, toss with the sea salt and then leave for about 30 minutes.
Roast the chillies and peppercorns in a preheated 200°C (400°F/Gas 6) oven for about 3 minutes, or until the chillies blacken and the peppercorns are fragrant. Pound the spices in a mortar with a pestle until coarsely ground.
Put the vinegar and sugar in a small pot over low heat and stir until the sugar dissolves. Bring to the boil, turn off the heat, then add the ground spices and mix together.
Firmly pack the salted cabbage into a 1 litre (35 fl oz/4 cup) sterilised glass jar (or several smaller jars). Gradually pour in the spice and vinegar mixture, allowing the liquid to find its way to the bottom of the jar before you add more. You should be able to add most, if not all, of the liquid. Seal with a tight-fitting lid and set aside for at least two days before using.