Mix all the marinade ingredients in a bowl, rub the mixture over the lamb, cover and chill overnight.
Remove the skins from the tomatoes by plunging them into a saucepan of boiling water, count to 12, then drain and refresh under cold running water. Preheat the oven to 110C/gas mark ¼. Cut the tomatoes in half lengthways, place them cut-side down on a wire rack on top of a baking tray. Sprinkle with a little olive oil, place a slice of garlic on each, season with salt and a pinch of sugar. Place in the preheated oven for four hours. Remove the tomatoes from the oven and leave to cool.