Mix the sausagemeat with one-third of the onions, half the thyme and plenty of seasoning. Shape into balls about 1.5 cm (¾ inch) in diameter.
Cut all the meat into small pieces. Season the plain flour and use to coat the meat. Melt 25 g (1 oz) of the butter in a large, flameproof casserole and brown the meat in batches, draining each batch to a plate.
Melt another 25 g (1 oz) of the butter and fry the remaining onions and the celery for 5 minutes. Add the garlic and fry for 1 minute. Stir in any remaining coating flour, then blend in the stock. Return the meat to the pan with the juniper berries. Cover and place in a preheated oven, 160°C (325°F), Gas Mark 3, for 1 hour until the meat is tender.
Meanwhile, put the self-raising flour and baking powder in a food processor with a little salt, the remaining thyme and the remaining butter, cut into pieces. Blend to breadcrumb consistency. Add most of the milk to make a dough, adding the rest if it is very dry. Turn out on to a floured surface and roll out to 1.5 cm (¾ inch) thick. Cut out rounds using a 4 cm (1¾ inch) cutter.
Stir the chestnuts and the Worcestershire sauce into the casserole and check the seasoning. Arrange the scones around the edge and glaze with milk. Raise the oven temperature to 220°C (425°F), Gas Mark 7, and cook for 20 minutes or until the scones are cooked through.