Season the flour with salt and pepper on a plate. Coat the venison with the flour.
Melt the butter in a flameproof casserole and fry the venison in batches until browned, lifting out with a slotted spoon on to a plate. Add the pancetta or bacon, leek, carrots and parsnips to the casserole and fry gently for 6–8 minutes until lightly browned. Add the garlic, rosemary and any flour left over from coating, and cook, stirring, for 1 minute.
Blend in the stout and stock and bring to a simmer, stirring. Return the venison to the casserole, then reduce the heat, cover and cook very gently for 1½ hours or until the meat is tender.
Add the chestnuts and potatoes and cook for a further 20 minutes or until the potatoes are cooked through. Season to taste with salt and pepper.