Combine the potatoes, celery, onion, ham and water in a stockpot and bring to a boil, then cook over medium heat until potatoes are tender, 10-15 mins. Stir in bouillon, salt and pepper. In a separate saucepan, melt butter, whisk in flour, stirring constantly until thick, 1 minute. slowly stir in milk, stirring until thick, 4-5 mins Add to stockpot slowly.