Wash the chicken thoroughly with cold water and pat dry with paper towels. Lay the chicken on a clean cutting board and sprinkle the skin and cavity with a pinch of salt and pepper and the lemon juice. Stuff the grapes, cilantro and garlic into the cavity. Tie the legs together with some kitchen string and tuck the wings behind the back of the chicken.
Mix together the fenugreek leaves, garam masala, cardamom, smoked paprika, turmeric, cinnamon and a pinch of salt and pepper in a small bowl. Loosen the skin on the breast of the chicken and rub some of the spice mixture under it, taking care not to tear the skin, and then rub the remaining mixture liberally over the rest of the chicken.
Lay the banana leaves flat on a counter top. Carefully wrap the marinated chicken in them, making sure it is completely covered. Using kitchen string, tie it snugly to hold the leaves in place. (If using foil, wrap the chicken completely in foil.) Place the chicken into a large roasting pan and place in the oven until the chicken is cooked through and the juices run clear, 1 to 1 1/2 hours. Allow to rest for 15 minutes before carving.