1. Remove all string from the beef and unroll. Lay the meat out flat in a non-metallic dish and coat completely with the steak rub. Cover and leave in the fridge for at least 12 hours, or for up to 24 hours.
2. Preheat the oven to 150°C, gas mark 2. Place all of the sauce ingredients into a large roasting tin big enough to hold the meat in a single layer. Season with black pepper only. Mix well, then add the beef and any juices to the tin. Spoon the sauce over the meat, then lay a piece of scrunched-up, damp baking parchment on top, followed by a large single piece of foil. Seal tightly around the edges of the tin. Place in the oven for 3 hours. Remove from the oven and leave covered in the tin to cool completely at room temperature.
3. Prepare and light the barbecue, then leave until the coals are white. Remove the meat from the sauce and place it onto the barbecue rack. Cook for 20 minutes, turning the meat very frequently, until piping hot throughout.
4. Meanwhile, bring the sauce in the roasting tin to the boil over a high heat and simmer vigorously for 20 minutes, or until thickened and reduced by about two-thirds. Strain through a sieve into a jug, pressing well with the back of a spoon to release all the juices. Serve the meat thinly sliced, with lots of sauce to spoon over.