Heat the oil and butter in a frying pan; add the leeks and fry until soft and cooked.
Add the pork mince, garlic, parsley, sage, salt, pepper, allspice, dried tarragon and cooked leeks to a bowl. Mix to combine without overworking the meat.
Preheat the oven to 200 ºC and line a baking sheet with baking paper.
Unroll the pastry and leave it on the plastic it was rolled in; cut it in half lengthways. Divide the pork mixture in two and spread along the length of each pastry strip in a cylinder shape, leaving a 1cm edge. Tightly wrap and roll the pastry around the meat; brush the ends of the pastry with the beaten egg and secure by rolling over and lightly pressing.
Cut the sausage rolls into 2 cm-sized portions. Arrange the sausage rolls on the prepared baking sheet. Brush more egg wash on the top of each sausage roll. Place the tray into the oven and bake for 25-35 minutes until the pastry is puffed and crisp and the meat is cooked through.
Combine the mayonnaise, mustard, honey and Worcestershire sauce in a bowl and stir to combine.
When sausage rolls are done baking, serve hot with the dipping sauce.