you want Pate Brisee for the best mince pies...basically it's fancy shortcrust.
I use block margarine or block butter, and one and a half times the weight of the butter in SR flour, cut the cold butter into small cubes and then rub it into the flour with your fingertips until it looks all crumby and with no floury pockets in it.
Then I add about half a cup of sugar...icing or caster....but icing is nicer (this is for one block 225g size of butter)
Get a knife and and break an egg into the pastry and then using the knife cut through the egg and crumbs until it all starts to combine, you may need a splash of water, but ime water makes the pastry hard so avoid it if you can
Gently form the pastry into a dough, do not knead or squeeze ...this will also make it tough
Let it rest for a while in the fridge before use.
** I know SR flour is technically wrong for pastry, but for mince pies it works
As far as the boiling over goes...I think mincemeat is too appley just now and this doesn't help, but make sure you seal the tops on well with egg and make sure you have a small hole in the top to let the steam out and that helps prevent the lids popping off.