Preheat oven to 160°C. Tie pork at 4cm intervals with kitchen string. This will shape neck into around roll. Heat oil in a large cast iron or heavy based pot over a high heat. Season pork, then add to pot. Cook, turning regularly, for about 8-10 minutes or until brown on all sides. Remove and set aside.
Add speck and sage to pot and cook, stirring, for 3 minutes. Remove and set aside. Add onion and cook, stirring, on a medium heat for 5 minutes or until soft. Return speck and sage to pot.
Pour in verjuice, stock and port and stir until combined. Return pork to pot and turn to coat in liquid. Scatter carrots, swede and parsnips around pork.
Cover with lid. Roast in oven for 2 hours, turning meat once during cooking. Transfer pork, carrots and parsnips to a serving dish, cover with foil and rest for 10 minutes.
Bring liquid in pot to the boil. In a small bowl, combine flour and 2 tbsp water to form a smooth paste. Add to pot and stir. Boil uncovered for 7-8 minutes or until thickened. Carve pork. Serve with vegetables and sauce spooned over.