Preheat the oven to 200°C/180°C fan/gas 6. Place the chicken legs in a roasting tin and rub the olive oil all over the skin. Place the rosemary and the lemon wedges over the chicken. Cover the chicken with a large sheet of foil and bake for 45 minutes, then remove the foil and cook for a further 15 minutes until cooked through. Leave to cool and rest for about 15 minutes.
Meanwhile, place the new potatoes in a medium saucepan, cover with water and cook for 10 minutes. Add the green beans to the pan with the potatoes and cook all together for a further 7 minutes. Drain the potatoes and beans and place in a large serving bowl. While still warm, add the spring onions and toss together well.
Remove the meat from the chicken legs and either chop of simply tear into bite-sized pieces. Add the chicken to the bowl with the potatoes and beans, tossing everything together well. Squeeze out the juice from the roasted lemons into the bowl, too.
Mix all the dressing ingredients together, then pour over the chicken and vegetables and toss together well. Spoon into a large rigid plastic container to take to the picnic. Serve with cherry tomatoes and lettuce.