Heat the oven to 150C/130C fan/gas 2. Mix the stock cube with the tarragon and pepper in a small bowl. Rub over the brisket and season. Heat the oil in a large flameproof casserole dish or roasting tin over a medium-high heat, add the brisket and brown well on all sides.
Stir in the ale and beef extract, then tip in the sugar and onion flakes and add enough of the beef stock to come two-thirds of the way up the sides of the brisket. Bring to the boil, cover and roast in the oven for 3 hrs, checking every 30 mins to see if the brisket is tender (but you may need to roast it for up to 5 hrs). Transfer the brisket to a board and leave to rest for 15 mins. Slice and serve with the stock spooned over and some steamed long stem broccoli on the side, if you like.