I had brisket at the weekend - about the same size of joint. My favourite method is slow roasting at 160C for about 4 hours, leaving it to cool a bit and slicing it thinly (leaving it to cool isn't essential to the cooking, it just makes it easier to slice neatly). I then dissolve a beef stock gel pot thing in enough water to cover the brisket slices in a casserole dish and then cook it for another 3 hours at 180C. Should come out lovely and juicy and tender and you can make gravy from the stock.