Put all the marinade ingredients in a mixing bowl and blend with a hand-held blender.
Massage the marinade into the lamb and let it sit in a ceramic dish for several hours or overnight.
Remove the lamb from the fridge 1 hour before cooking to allow it to come to room temperature.
Preheat the BBQ grill. When hot, place the lamb flesh side to sear for a couple of minutes. Then turn over, skin side down. Turn the heat down, and cook the lamb for 15-20 minutes. The lamb is ready to turn over when it no longer sticks to the grill.
Turn the lamb over, flesh side down, and cook for 15-20 minutes. When cooked to medium rare, the lamb will be a lovely pale pink inside.
Set aside for 10 minutes. When ready to serve, thinly slice, arrange on a platter and pour lamb juices over the top.