1. Preheat the oven to 220 ̊C, gas mark 7; remove the beef from the fridge at least 1 hour before cooking. Mix the mustard powder, sugar, salt and pepper, and rub all over the beef. Put the onion, garlic, celery, carrots and rosemary in a large roasting tin and sit the beef on top. Roast for 20 minutes, then lower the heat to 160 ̊C, gas mark 3, and roast for another 2 hours 30 minutes-3 hours for medium-rare. A thermometer inserted into the centre should read 55-60 ̊C.
2. Transfer the joint to a carving board and cover with foil; rest for at least 1 hour – the perfect time for roasting your potatoes.
3. For the gravy, pour off the excess fat from the roasting tin, then place on the hob over a medium heat. Pour in the port and stock and bubble for a few minutes, using a wooden spoon to scrape up any sticky bits from the base of the pan. Tip everything into a saucepan with the blackberries and juniper. Simmer briskly for about 20 minutes, then pass through a sieve, pushing gently to extract the juices, and return to the pan.
4. Carve the beef (it's easier if you remove the bones first). Add any meat juices to the gravy, simmer again briefly, then pour into a jug to serve with the roast.