First prepare the garlic for the mash. Cut the head of garlic in half horizontally and place both halves, cut side up, on a sheet of tin foil. Drizzle with half of the rapeseed oil and season with sea salt and black pepper. Loosely wrap the foil over the top of the garlic to make a parcel and twist the edges to seal. Place on a baking sheet and roast in the preheated oven for 30 – 40 minutes, until the garlic is completely soft and lightly caramelised.
For the sausages, heat the rapeseed oil in a casserole dish or heavy based saucepan and brown the sausages well on all sides. Remove from the pan and set aside. Add the onion and cook over a low heat for 10 minutes until soft and translucent. Add the juniper berries and flour and cook for a further 2 minutes. Blend in the balsamic vinegar, red wine and stock, stirring all the time, until you have a smooth, thickened sauce. Add the redcurrant jelly and herbs and return the sausages to the pan. Simmer gently for 15 minutes.
For the mash, place the potatoes in a large pan of salted water and bring to the boil. Simmer until tender, drain very well and purée through a ricer for a very smooth texture. If you do not have a ricer just ensure that you mash the potatoes as smoothly as possible. Season well with sea salt and black pepper and squeeze in the roasted garlic cloves. Add the remaining rapeseed oil and beat in well. Serve with the sausages and sauce.