Heat the oven to 200C/180C fan/Gas 6 and line a large baking tray. Squeeze the sausage meat out of the skins into a bowl, add the mustard if using, a pinch of salt and plenty of black pepper, and squish everything together with your hands.
Unroll the pastry and cut the sheet in half lengthways to make two long strips. Divide the sausage meat in half and roll each portion into a long snake the length of each strip.
Lay one snake down the centre of each strip, brush one long edge with beaten egg, then fold the pastry over the filling and press firmly to seal.
Cut each long roll into four pieces so you have eight sausage rolls in total. Place them on the tray, glaze all over with the remaining egg, and cut two small slits in the top of each.
Bake for 20-25 minutes until deep golden, puffed and cooked through. Rest for 5 minutes, then serve warm or cold.