Peel celery and grate finely. Mix celery in a bowl with half of lemon juice. Rinse apples, cut into quarters, remove seeds and cut into small cubes. Coarsely chop walnuts. Mix mayonnaise in a bowl with double cream and remaining lemon juice and season with 1 pinch of salt and 1 pinch of sugar. Mix celery, apples and walnuts in a bowl. Arrange lettuce leaves on plates, cover with celery mixture, drizzle with mayonnaise mixture and serve.