1) In a large saucepan over high heat, combine the water, cider vinegar, red wine vinegar, onion, carrot, salt, pepper, bay leaves, cloves, juniper and mustard seeds. Cover and bring this to a boil, then lower the heat and simmer for 10 minutes. Set aside to cool.
2) Pat the bottom round dry and rub with vegetable oil and salt on all sides. Heat a large saute pan over high heat; add the meat and brown on all sides, approximately 2 to 3 minutes per side.
3) When the marinade has cooled to a point where you can stick your finger in it and not be burned, place the meat in a non-reactive vessel and pour over the marinade. Place into the refrigerator for 3 days. If the meat is not completely submerged in the liquid, turn it over once a day.
4) After 3 days of marinating, preheat the oven to 160C/Gas 3.
5) Add the sugar to the meat and marinade, cover and place on the middle rack of the oven and cook until tender, approximately 4 hours.
6) Remove the meat from the vessel and keep warm. Strain the liquid to remove the solids. Return the liquid to the pan and place over medium-high heat. Whisk in the gingersnaps and cook until thickened, stirring occasionally. Strain the sauce through a fine mesh sieve to remove any lumps. Add the sultanas if desired.
Slice the meat and serve with the sauce.