First brine the beef. Make up a 3% brine solution. The easiest way to do this is to put your beef in a container (I use a large lunchbox), cover with water and remove the beef again. Pop on the scales and weigh the container + water. Work out 3% of this (the weight multiplied by 0.03) and add that much salt. Stir well, pop the beef back in and cover. Leave for five days.
After five days, remove from the brine and pat dry. Preheat a water bath to 68C. Mix the sugar, paprika and mustard and rub all over the beef. Seal in a vacuum bag and cook sous vide for 24 hours.
After 24 hours remove the beef from the bag but reserve the liquid. Pop this liquid into a pan, pop on a high heat and reduce by half. Stir in a tablespoon of your favourite BBQ sauce (I'm digging the Tesco Louisiana at the moment)
Meanwhile, blowtorch the outside of your brisket to give a smoky char. Slice thickly and dunk into the sauce, before serving in a toasted bun with pickled gherkins and salad, with extra sauce on the side.
Notes
If you don't have access to a sous cide machine, you could do this in a low slow cooker for 12 hours. If you don't have a blowtorch, finish on the BBQ or in a hot pan.