Place the joint in a large saucepan and cover with cold water. Bring to the boil, then simmer gently for approx. 20 minutes per half kg.
Set oven to Gas Mark 6, 200C (400F). Remove the joint from the saucepan. Reserve liquid.
Remove the rind and score the fat. Place the joint on a roasting dish. Spread with the mustard and brown sugar.
Add approx.125ml of the reserved liquid to the roasting dish (keep the remaining liquid for soup). Place in oven for 20 minutes. Top up the liquid if necessary during cooking.
Pour off the juices from the joint. To this add the redcurrent jelly, port and redcurrants. Simmer until you have a good rich consistency. Carve the joint and serve with sauce.