Cook the broad beans in boiling salted water for 1 minute. Drain, rinse under the cold tap and peel off the skins. Simmer the French beans in salted boiling water for 4-5 minutes, until just cooked. Drain, rinse, then cut into 2.5cm lengths.
Mix the beans in a large bowl with the chickpeas, tomatoes and herbs.
To make the dressing, whisk the lemon juice with the sugar, tomato purée and salt and pepper, then gradually whisk in the olive oil. Pour over the salad and toss together. Serve at room temperature.