Rub the duck legs with the coarse sea salt, freshly ground black pepper, one crushed garlic clove, four sprigs of thyme and the bay leaves. Pack tightly into a ceramic dish, skin side down, then drizzle over the brandy, cover the dish with cling film and leave to marinate in the fridge for 24 hours.
Preheat the oven to 150C/300F/Gas 2.
Scrape the marinade off the duck pieces.
Heat the duck or goose fat in a heavy ovenproof casserole dish, then add the duck, the remaining garlic cloves and the remaining thyme. Make sure that the duck is completely submerged in the melted fat.
Transfer the dish to the oven and bake for 3½-4 hours, or until the meat is very tender when pierced with a skewer and the fat in the skin is completely rendered.
Remove from the oven and allow to cool uncovered. Strain half of the fat and bring to the boil in another pan, then pour into a clean ceramic dish large enough to hold the duck without it touching the sides. Leave to set in the fridge.
Once the duck is cool, place into the prepared ceramic dish and strain over the remainder of the melted fat and place into the fridge for 1-2 hours to set. (This can be stored in the fridge for up to one month.)
Preheat the oven to 200C/390F/Gas 6.
Scrape off the fat from the duck legs, then place into a roasting tray. Sprinkle over the coarse sea salt and roast in the oven for 40 minutes, or until the skin is crisp and golden-brown. Serve immediately.