For pastry: Combine flour, suet, baking powder, salt and water to form a smooth dough. Dust a smooth surface with flour and press out pastry with fingers to form a circle 1/4 inch thick. Lay a pudding cloth or cheesecloth into a basin or large ovenproof bowl and mold the pastry to it. In a separate bowl, combine kidney and steak. Season well with salt and pepper. Add parsley and thyme. Dust with a little flour.
Place half the meat mixture into the suet crust. Add half the mushrooms, then the remaining meat, and the mushrooms again. Add enough oyster juice to the wine to make 1/2 cup and pour into the pudding.
Brush the edges of the crust with water. Use the cloth to fold the pastry sides over to cover the filling. Cut away excess dough. Tie cloth with a string.
Bring a pot (large enough to completely enclose the pudding bowl) filled half way with water to a boil. Place pudding bowl in pot-the water should reach up to 3/4 of the sides of the bowl. Cover with lid. Or to mold the pudding to the shape of a baby's head, remove the nob from the lid of the pot and thread the cloth's string through the exposed holes. Secure the string to the pot's handles to suspend the pudding in the pot. Boil pudding for 3 1/2 hours. Make sure the water doesn't boil away. One hint: place two marbles in the pot with the water. Their pinging will notify you when the water completely evaporates.
Remove pudding from bowl. Carefully pull away pudding cloth and invert pudding onto a large, heated serving dish. Cut out a top in the pudding's crust and add the oysters, mixing them in with a fork.