Preheat oven to 375 degrees. Have a 9 or 10 inch removable bottom tart pan ready. To make the crust combine the flour, sugar and pinch of salt in a bowl. Cut in the margarine or butter with your fingers or a pastry blender until the mixture resembles coarse crumbs. Add sherry and stir with a fork until the dough can hold together. Add a few drops of water if necessary. Press the dough evenly into the tart pan, pressing dough up sides. Refrigerate crust while preparing filling. Filling: Place the plums, sugar, lemon zest and flour in a large bowl and toss gently to blend. Remove crust from refrigerator. Spread with currant jelly or apricot jam (warm the jam in the microwave if necessary to make it spreadable.) Arrange plums in the crust with cut side up. Sprinkle sherry over the plums. Bake at 375 degrees for 50 minutes, or until the crust is deep brown and the filling bubbles.