Mash the pepper, salt, 2 garlic cloves, juniper and 1 rosemary sprig in a pestle and mortar.
Cut the grouse in half using poultry scissors.
Rub the salt mix roughly over the grouse skin and put them skin side down in a large flat dish. Pour the sloe gin over the grouse, cover with cling film and leave overnight.
Remove the grouse from the dish and brush off the salt mix using some paper towel.
Place the grouse (skin side up), the remaining garlic and rosemary sprigs in a large oven dish and cover with duck fat (ensuring the grouse is completely covered).
Put the dish in the oven (160°C, Gas 2) for 3-4 hours. The meat should fall off the bone easily when touched.
Remove the grouse and the garlic from the baking dish and place the grouse skin sideup on a baking tray.
Roast in an oven at 190°C, Gas 6 until the skin crisps (roughly 30 minutes).
Serve with the garlic cloves, some salad,and rustic potatoes or chips.