Preheat the oven to 160°C/fan 140°C/gas mark 3. Heat the oil in an oven-proof casserole dish over a medium heat. Toss the venison pieces with the flour and season well with black pepper. Cook the meat in batches until browned all over. Remove from the pan and set aside.
Add the pancetta to the pan and cook for 3-4 minutes, until starting to crisp. Add the onion, celery and carrot and continue to cook over a low to medium heat for 8 minutes, until starting to caramelise. Add the garlic and thyme and continue to cook for a further 1 minute. Return the venison to the pan. Add the chestnuts, port and chicken stock. Bring to a simmer and cover with the lid. Transfer to the oven for and cook for 1¼ hours.
Meanwhile, cook the potatoes until tender. Drain and leave to steam in the pan for a couple of minutes. Add the milk and horseradish and mash until really smooth. Set aside.
Remove the casserole from the oven and increase the temperature to 180°C/160°C/gas mark 4. The meat should be tender and the liquid reduced. If it is still quite liquid, return the casserole to the hob and bubble until the liquid has reduced. Season with black pepper.
Put the potato in a piping bag with a star nozzle and pipe swirls of potato to completely cover the top. Return to the oven for 20-25 minutes, until golden on top.