The night before baking, freeze the butter for the pastry. On the day, tip the flour into a big bowl with a pinch of salt and the mustard powder. Hold the butter block in its wrapping and coarsely grate straight into the flour, peeling back the wrapping a little at a time as needed and dipping the end of the butter into the flour every so often to stop it clumping in the bowl. Use a palette or cutlery knife, and lightly stir together, then stir in about 200ml ice cold water and the vinegar to bring the dough together. Wrap and chill for at least 30 mins.
Meanwhile, make the filling. Tip the sausagemeat into a bowl with the stuffing mix, 100ml cold water and lots of black pepper. Squish everything together with your hands until evenly blended, then chill in the fridge while you roll out the pastry. Tip the dough onto a floured surface, roll out to a rectangle about 50cm x 25cm and trim the edges. Brush the pastry lightly all over with some of the egg, then mould the sausage mixture into a 50cm roll. Sit the sausage just off-centre on the pastry, then fold the pastry over and crimp or use a fork to press the ends together. Slice into eight pieces using a knife dipped in warm water and wiped clean between slices. Place on a baking tray and chill for at least 30 mins. Will keep chilled for up to to 24 hrs. To bake sooner, put in the freezer for 15 mins.
Heat the oven to 220C/200C fan/gas 7. Brush the rolls with more beaten egg, bake for 20 mins, then brush again and return to the oven for a further 10-15 mins until light mahogany in colour. Leave to cool on a wire rack, then serve warm with English mustard, ketchup or brown sauce and a chilled glass of cider, if you like.