Take the joint out of the fridge well ahead of time so it loses some of its chill.
Put the rub ingredients in a mini chopper or a pestle and mortar and blend them to a smooth, even paste.
Remove the sinew from the underside of the joint, then score the fat cap on top in a diamond pattern. Cut through the fat only, not down into the meat.
Pat the joint dry with kitchen paper, then rub the paste well down into the scored fat on top and over the underside of the joint.
Insert a meat thermometer into the joint and cook it over indirect heat at about 190°C in a closed barbecue, lid on.The barbecue should be hot when the meat goes on.
Cook until the core temperature reaches about 52-53°C, then take the joint off and let it rest on a chopping board for at least 15 minutes.The thickest part will be pink, with the ends more well done. The rump cap is at its best served at a core temperature of 57-59°C.If you prefer it more done, leave it on the barbecue until the core temperature is about 56°C, then rest it for at least 15 minutes until it reaches 60-63°C.
Carve the joint into slices, starting at the thin end and slicing until you reach the middle of the joint. When the grain of the meat changes direction, turn the joint and carve from the side.