Rinse and pat dry the flank steak. Score the meat in a diamond pattern, to allow the flavour of the marinade to penetrate.
In a bowl, combine all the ingredients other than the flank steak, mix well.
Pour the marinade into a flat dish or large Ziploc® bag. Place the meat in the marinade and cover/seal. Place in the fridge for 6 hours or overnight, turning 2-3 times.
Preheat the BBQ on high, and brush the grill with oil to prevent sticking. Place the flank steak on hot grill, reduce heat to medium. Grill approximately 5 minutes per side for medium. Allow the steak to rest for 3-5 minutes before carving.
Meanwhile, place the marinade in a small heavy saucepan and bring to a boil. Cook for 2-3 minutes.
Carve the steak into thin slices on the diagonal, against the grain. Drizzle cooked marinade over the meat.