Put the onion, carrots, celery and beef together in a large non-stick saucepan and dry-fry over a high heat for 2–3 minutes, until the beef is no longer pink. Keep stirring with a couple of wooden spoons to break up the mince.
Reduce the heat and add the ketchup, brown sauce, Worcestershire sauce, flour, chilli powder and chilli flakes (if using) and black pepper.
Cook for a minute, then add the potatoes and stock.
Bring to the boil, reduce the heat and cover loosely. Simmer for 25 minutes until the potatoes are tender, stirring occasionally. If necessary, remove the lid for the last five minutes and stir more regularly until the mixture is thick.
Remove from the heat, stir in the frozen peas and leave to cool completely.
To make the pastry, put the pizza base mixes in a large bowl and mix with lukewarm water according to the packet instructions.
Turn out on to a lightly floured surface and knead for five minutes until the dough is smooth and elastic.
Preheat the oven to 180C/350F/Gas 6.
Using scales, divide the dough into six even portions – each will be about 80g/3oz.
From each portion, take off 35g/1¼ oz of the dough to use for the pie lids.
Roll out the larger piece of each portion on a lightly floured surface until it is about 3mm/ 1/8in thick and large enough to line your foil pie dish, leaving a little overhanging the sides.
Lift into a pie dish and press down well into the base and sides. This can be a little tricky first time round, but the dough is very forgiving, so take it slowly until you’ve got the hang of the method.
Spoon a sixth of the mince and potato mixture into the pie case. Brush the overhanging edges with a little milk.
Roll out the smaller portion of pastry until it is large enough to cover the pie dish. Lift it on top of the pie and pinch the pastry firmly together. Trim with kitchen scissors – using scissors will stop the dough stretching.
Repeat exactly the same process to make the other five pies and place them on a baking tray.
Snip once in the centre of each pastry lid with scissors to create a large hole for steam to escape. Brush the pies generously with more milk.
Bake the pies for 15 minutes, then remove them from the oven and cover each one fairly tightly with foil.
Put them back in the oven for a further 15 minutes or until the pies are pale golden brown and the filling is piping hot.
Remove the pies from the oven and leave to stand for five minutes before removing the foil, then tuck in!