Place each banana on a double piece of foil.
Brush well with the lemon juice and sprinkle 1 tablespoon sugar on each.
Cream together the butter and cinnamon and divide between the bananas, dotting along the top.
Wrap each banana securely and cook on the barbecue grid, about 10 cm (4 inches) above the coals, for about 10 minutes.
Unwrap and serve hot, on the foil.