In a medium bowl, whisk the first 8 ingredients to make the marinade. Remove 1⁄2 cup of marinade and refrigerate to be used for basting. Coat the lamb with the remaining marinade. Refrigerate mixture and leave to marinate for 2hours. Once nicely marinated, thread lamb onto skewers alternating sweet peppers and onions. Grill or barbaque over a medium-high heat, turn occasionally to prevent burning and brush with reserved marinate on each turn.