Heat oven to 200°C (180°C fan-forced). Line a baking tray with baking paper.
Bring a saucepan of water to the boil, add potatoes and cook until tender. Refresh under cold water and slice into 1cm cubes.
Meanwhile, heat 1 tablespoon of olive oil in a sauté pan over a low heat. Add leeks, garlic and thyme, stirring until combined. Cover and cook for 10 minutes. Remove from heat and place in a bowl to cool.
Add another tablespoon of olive oil to the same pan over a medium heat. Add the bacon and cook for 5 minutes.
Remove from heat and add to the bowl with the leeks, leaving the rendered bacon fat in the pan. Add the cooked potatoes cubes to the pan with the remaining olive over a medium high-heat and cook for 5 minutes or until golden. Remove from heat and allow to cool.
Place the leek mixture in a large mixing bowl with the Neufchatel, cheddar, chives, mustard, salt and pepper, stirring until well combined. Add the potatoes and fold through until combined.
Place the pastry sheet on a lightly floured work surface. Cut two rectangles, one 12 x 26 cm, for the base and the other 14 x 26 cm, for the top. Roll the rectangle for the base into a 20cm x 32cm rectangle and place on prepared baking tray. Roll the rectangle for the top into a 22cm x 34cm rectangle.
Spread the filling down the center of the prepared base, leaving a 2cm border. Brush the edges with egg glaze and carefully place the prepared pastry top over the filling, to encase it. Push gently around the border to seal in the filling. Using a fork gently press the edges to seal the pie and score the top with a sharp knife.
Brush the top of the pie with egg glaze and place in pre-heated oven for 15 minutes. Reduce oven temperature to 180°C (160°C fan-forced) and cook for a further 30 minutes. Remove from oven and allow to cool for 15 minutes, before serving.