In a medium-sized pot, bring the lentils and water to a boil. Turn the heat down to a gentle simmer. Add celery, leek, carrot, bay leaf, thyme and vegetable stock powder (if using). Simmer for 15 – 20 minutes and then add the cubed potatoes. Simmer until potatoes are cooked, then season to taste with salt, pepper and maybe more vegetable stock powder and water if required. At this stage, remove the stalks of thyme if using fresh thyme. If more water has been added, bring to a boil again. Turn the heat off and let it sit for 5 minutes. Serve.