Put half the pork shoulder, half the pork belly, half the bacon, the liver and garlic in a food processor and blend until smooth. Add the remaining pork shoulder, pork belly, bacon and pulse until roughly chopped and well combined.
Transfer to a bowl and stir in the orange juice and zest, brandy and herbs until well combined. Grind the peppercorns and juniper berries together with a pestle and mortar and add to the mixture. Stir until well combined. Cover the bowl with clingfilm and chill in the fridge for 1-3 hours.
Place the smoked bacon rashers on a board and stretch one at a time with the back of a knife. Place the first rasher diagonally in the terrine mould making sure some overlaps the edge. Place the second rasher diagonally in the opposite corner. Repeat with the remaining rashers, changing the angle slightly with each rasher so that the whole terrine mould is covered with bacon. (This will make the terrine easier to cut.)
Preheat the oven to 150C/300F/Gas 2. Spoon a third of the pâté mixture into the lined terrine and place half the cornichons neatly on top. (They need to run lengthways down the terrine, so that when the terrine is cut, they form a row of green circles in the centre.)
Spoon another third of the pâté mixture on top of the cornichons and press the surface smoothly and firmly. Cover with a second layer of cornichons and finish with the remaining pâté. Bring the overlapping bacon up and over the pâté to cover the surface.
Cover with the lid (or aluminium foil if using an ovenproof dish) and place the terrine into a small roasting tin. Add enough just-boiled water to the roasting tin to come 2cm/¾in up the outside of the terrine. Bake in the centre of the oven for 1½ hours.
To test the terrine is ready, remove from the oven and insert a skewer into the centre. Hold for 10 seconds, then remove and lightly touch the end. The skewer should feel hot. The pâté should also have shrunk away from the sides of the terrine. Remove from the roasting tin.
Remove the terrine lid, cover with a double layer of aluminium foil and place a couple of cans of beans or some other heavy weights on top. Leave to cool, then chill in the fridge overnight. The next day, turn the pate out onto a board and cut into thick slices. Serve with hot, crusty bread and lots of butter.