Put the lamb in a bowl, season liberally with salt and pepper, then pour over half the lemon juice. Stir well and leave to marinate for 10 minutes.
Meanwhile, peel and halve the cucumber and remove the seeds with a teaspoon. Roughly dice the cucumber and mix it with the remaining lemon juice and half the yogurt. Set aside.
Pour the remaining yogurt over the lamb and mix well.
Lay the lamb on the barbecue grid and cook for 5 minutes - the underneath should look quite dark as the yogurt cooks. Turn the lamb over and cook for a further 5 minutes, then place on a board and leave to rest while you heat the naan on the barbecue. Slice the lamb into pencil-thick strips, then cut the naan in half. Spoon in the cucumber sauce and top with the lamb and its juices.
