Grease and line a large baking sheet with greaseproof paper.
Boil the potatoes until they are soft enough to mash; mash them until they are smooth, then add the mustard powder, crumbled Stilton cheese and the diced celery.
Season to taste with salt and pepper and set aside.
Roll the pastry out on a well-floured board and stamp out circles with a saucer or small plate, to about 5” (13cms) in diameter.
Spoon some of the filling mixture over one half, moisten the edges of the pastry then fold the pastry over to form a crescent and seal by crimping around the edges with your fingers or a fork.
Place the pasties on the baking sheet, cut a slit to allow the steam to escape and brush them all over with the beaten egg and milk mixture.
Bake for 20-25 mins until they are golden brown.
Remove the pasties from the oven and allow to cool slightly before serving warm, or cold.
Can be frozen before baking.
Recipe by appetite magazine at https://www.appetitemag.co.uk/stilton-cheese-and-fenland-celery-pasties/