Remove the rump cap from the fridge at least 1 hour before cooking to bring it to room temperature. Preheat the oven to 180°C (gas mark 4) on the conventional setting.
Using a sharp knife, score the fat cap in a criss-cross pattern, taking care not to cut into the meat.
Rub the joint all over with coarse sea salt and freshly ground black pepper.
Place the rump cap, fat-side up, on a roasting tray (use a wire rack if available) and drizzle over a little extra virgin olive oil.
Roast for 45–50 minutes for medium (pink in the centre). For well done, roast for a further 10–15 minutes, until the fat is golden and crisp.
Remove from the oven and transfer to a board. Cover loosely with foil and leave to rest for 10–15 minutes.
Carve into thick slices against the grain and serve with roast potatoes, a fresh salad or grilled vegetables.