Slice the venison into roughly 2.5cm/1 in pieces. Put the flour into a large bowl, add the sliced venison and stir around to coat all over with flour. Peel and slice the onions and cut the mushrooms in half. Melt 40 g/1½ oz of the butter over a medium heat in a large casserole. Add the sliced onions and fry until dark brown, then add the mushrooms and continue frying for a few minutes until browned. Using a slotted spoon, remove the onions and mushrooms from the pan and keep on a plate on one side.
Preheat the oven to 240°C/475°F/gas mark 9. Add the remaining butter to the casserole, then add the floured venison and turn around to brown the meat all over. Stir in the ground spices, followed by the juniper berries, the onions and mushrooms and any leftover flour from the bowl. Finally stir in the marmalade, Guinness and soy sauce and season with a little salt and plenty of black pepper. Increase the heat and bring the juices up to boiling point, stirring around several times as you do so. Allow to bubble for 2-3 minutes while stirring, then, cover the dish and put into the oven. After about 10 minutes, turn down the heat to 150°C/300°F/gas mark 2. Cook for 1 hour.
Meanwhile, cut the skin from the belly of pork and slice the flesh into small pieces. When the venison has been cooking for 1 hour, add the pork and cook for another 30 minutes. Check for seasoning, then leave until cold.
To make the crust, mix together the flour, breadcrumbs, mustard seeds, if using, and suet. Season with salt and black pepper. Beat the egg with a fork in a measuring jug and make it up to 175 ml/6 fl oz with water. Gradually stir the liquid into the flour and breadcrumb mixture to form a soft, elastic dough. Gather up the dough and cut off a quarter of it to use for a lid.
Generously butter a 1.2 litre/2 pint pudding basin. Form the larger piece of dough into a ball and roll it out fairly thinly on a floured surface into a large circle. Fold the circle in half and then in half again. Lift into the prepared basin with the pointed end of the pastry downwards. Unfold in the basin, gently pressing it against the sides and leaving it hanging over the edge.
Spoon in the cooled venison filling and fold the overlapping edges of pastry back over the filling. Then roll out the remaining pastry into a circle big enough to form a lid. Moisten the pastry edges and place the lid on top, pressing the edges together to seal.
Cover the basin with a piece of buttered greaseproof paper, then a piece of foil. Secure with string under the rim but leave the paper and foil loose enough to puff up a bit. Make a string handle to lift up the pudding. Put the basin into a large saucepan and add enough boiling water to come half way up the sides. Cover the saucepan and simmer for about 2 hours, topping up with boiling water if necessary. Then lift out the basin, cut off the string, remove the paper and foil and serve the pudding straight from the basin.