To make pastry: In a medium bowl, sift together flour, baking powder and salt. Rub in suet until mixture resembles coarse crumbs. Stir in milk, a little at a time, and mix well. Knead briefly. Set aside.
For filling: In a medium bowl, stir together cherries, sugar, cinnamon and lemon zest until well mixed.
On a floured surface, roll the pastry into a rectangle 5mm thick. Brush with melted butter. Spread the cherry filling evenly over the surface of the pastry, leaving 1cm around the edges. Roll up the pastry, starting at a short end, and pinch together all the edges so that the juice will not escape. Wrap roll in a kitchen towel or muslin and tie loosely.
Place in a steamer over boiling water, cover and steam for 90 minutes, until pastry is firm. Remove cloth, slice and serve.