Squeeze the sausages out of their skins, and roll the sausagemeat* into 12 bite-size balls.
Heat ½ tablespoon of the oil in a 20cm ovenproof frying pan. Add the sausage and fry all over for 1 minute, until browned and cooked through. Add the bacon and mushrooms and fry for a few minutes, until golden. Add the tomatoes for a further minute. Remove the mixture and set aside. Discard excess fat from the pan.
Whisk together the eggs and chives and season with freshly ground black pepper. Stir in 2/3 of the fried sausage mix. Preheat the grill to medium.
Heat the remaining oil in the frying pan over a medium heat. Add the egg mixture, tipping the pan so that it covers the base. Turn down the heat and scatter over the remaining sausage mixture. Cook for 8 minutes, or until almost set.
Place under a preheated grill, for 2-3 minutes, until the frittata is set and golden. Let cool slightly, then slide out of the pan, cut into wedges and serve.