Peel, dice and cook potatoes, set aside. Melt margarine in a 6 quart pan, over medium heat. Add onion and green pepper, cook about 4 minutes, stirring frequently. Add flour, cook about 1 minute, stirring constantly. Gradually add milk, using a whisk to prevent lumping and to blend the sauce. Add corn, salt, thyme and pepper. Reduce heat to low, cook until the mixture appears hot, but not bubbling. Cut crab into bite-size pieces and add to the sauce. Add potatoes. Heat again until hot, but not bubbly, stirring frequently. Just before serving, add cooking sherry.