Place the dried peas in a suitable bowl. Cover with water. Leave without lid for one night.
Cut the leek into 2 cm long pieces, the turnip and the carrot into 1 cm cubes and the celery stick into slices.
Drain the peas well. Place in a large, heavy saucepan with the pearl barley, lamb and water and bring to the boil. Skim, add leek and turnip. Simmer gently at low temperature with lid on for 1 1/2 hours.
Add carrot and celery. Do without lid for another half hour.
Add white cabbage, stirring until cabbage is tender. Season soup to taste. Fold in parsley just before serving.