Combine the marinade ingredients with a little sea salt in a large bowl and mix well. Add the lamb to the bowl and turn a few times to coat well in the marinade. Cover and refrigerate for 6 hours, or overnight, turning occasionally. Remove the lamb from the fridge several hours before cooking.
Preheat the barbecue to hot. Brush the vegetables with a little of the oil and season with sea salt. Chargrill until golden on both sides, then place in a bowl to keep warm. In a mortar, pound the chilli and a little more sea salt with a pestle. Gradually incorporate the remaining oil and lemon juice to form a dressing and season with freshly ground pepper.
Meanwhile, place the lamb on the barbecue and pour over any marinade left in the bowl. Cook for 8 minutes, then turn over and cook for a further 8 minutes. Remove to a plate and allow to rest in a warm oven for 10 minutes.
To make the yoghurt, combine all the ingredients in a bowl and season with salt and pepper to taste.
Dress the vegetables with the dressing in the mortar and divide among four large plates. Carve slices of the lamb and top the vegetables with the lamb slices. Add a dollop of yoghurt, a squeeze of lemon and spoon over the juices from the resting plate.