Place the potatoes into a large pan of boiling water. Bring to the boil again, then simmer for 15-20 minutes until you can poke a fork through them without much force. Drain and set aside.
In a large pan, add the garlic infused oil and place over a medium heat. Once heated, add the chopped leek or onion and fry for 2 minutes.
Add the carrots and courgette, then fry for 4 minutes. Add the smoked paprika and cornflour, mix in, then fry for a further minute.
Add the chopped tomatoes, stock, dried mixed herbs and drained butter beans. Season with salt and pepper. Mix well and bring to the boil, then simmer and reduce for 10 minutes until you achieve a sauce-like consistency.
Preheat the oven to 220C/200C Fan/425F.
Whilst the pie filling is reducing, the potatoes should now be done. Once drained, return the cooked potatoes to the large saucepan and add the butter, milk, grated cheese and salt and pepper to taste.
Use a potato masher to mash until super smooth - it should be a little 'looser' than you'd usually serve mash so it's easier to spread on top of the pie. If yours is a little thick and stodgy, just gradually add a little more milk and mix in until easily spreadable.
Transfer the pie filling to a large roasting dish (mine is 13" x 10") and spread out into a flat, even layer.
Top with the mash and spread out into a flat, even layer. Use a fork to create lines all over the top of the pie and top with the remaining grated cheese.
Place onto a baking tray, then place into the oven for 20-30 minutes or until the top is golden and crisp.