For the pancakes whisk together the flour, eggs and milk and season with salt and freshly ground black pepper. Heat the oil in a frying pan then pour in a ladle of the batter. Swirl the batter around the pan to make a thin pancake and cook until bubbling. Flip the pancake with a palette knife, cook for one more minute, then transfer to a warmed plate. Repeat with the remaining batter; you will need six pancakes in total.
For the venison, trim the meat and keep any trimmings. Wrap in clingfilm and roll into a tight cylinder, leave to chill for at least half an hour. When ready to cook remove from the fridge and allow the meat to get to room temperature. Heat the olive oil in a large frying pan, season the venison with sea salt and freshly ground black pepper and sear in the hot pan until browned all over. Remove the meat from the pan and leave to rest.
Melt the butter in the frying pan and gently fry the mushrooms until golden-brown. Leave to cool slightly, then chop finely and set aside.
Add the shallots and garlic to the pan and cook gently until soft, but not brown. Add the chopped mushrooms and turn up the heat. Fry quickly until all the liquid has evaporated and the mixture is dry and begins to take colour. Season to taste with salt and freshly ground black pepper, then remove from the heat and set aside.
For the sauce, heat some olive oil and butter in a pan and gently cook the onion, carrot and celery until soft but nor brown. Add the meat including any trimmings fromt he venison and brown of at a high heat before adding the wine/port and reducing to a thick syrup. Add the chicken stock and ketchup and reduce again until you have reduced by 2 thirds. Strain the sauce and place in a saucepan, place on a medium heat and whisk a knob of butter through the sauce.
Preheat the oven to 180C/350F/Gas 4. Lay the pancakes out, overlapping them slightly so that there are no gaps, then spoon half of the mushroom mixture on top. Place the venison on top of the mixture and spread it with the remaining mushroom mixture. Trim the edges of the pancakes, then wrap the whole parcel up tightly.
Roll out a rectangle of pastry large enough to wrap the venison and about 3mm/⅛in thick. Place the venison in the centre of the pastry. Brush the edges of the pastry with the beaten egg and milk then pull the sides of pastry up to seal the venison in a parcel. Place the parcel on a non-stick baking tray, brush with the remaining beaten egg and milk, then bake in the oven for 30 minutes until the pastry is golden-brown and the meat is cooked through.
Rest for 10-15 minutes before serving.