Separate the stalks of celery and scrub well. Remove any strings from the larger stalks and slice the celery finely. Trim the base and excess green tops from the leeks, slit and wash thoroughly. Shred the leeks and add to the celery. Wash and slice the tomatoes and wash the watercress.
Mound the mixed leeks and celery in the centre of a flat serving dish. Arrange a ring of tomatoes around the edges and tuck the watercress heads under the tomatoes.
Serve the salad with the dressing separately, to spoon over as it is taken from the plate.