You can use ready made puff pastry to make this pithivier, but it tastes so much better if you make your own puff pastry. You can use this recipe to make your puff pastry, it will make more than you need, but you can freeze the extra.
You can make your own puff pastry, using this recipe. It will make more pastry than you need, but it freezes really well.
If you cannot get hold of Barber's 1833 Vintage Reserve Cheddar you can substitute it for another vintage or mature cheddar.