Preheat your oven to 200°C/Fan 180°C/Gas 6. Scrub and peel the potatoes then chop them into roughly equal-sized chunks, around 3-4cm across. Tip into a colander and rinse well with cold water.
Tip the potatoes into a large pan and pour in enough cold water to cover them. Add 1 tbsp salt, pop on a lid and set the pan on a high heat. Bring to the boil. When the pan is boiling, turn the heat down to medium and simmer for 4 mins.
While the potatoes simmer, add 4 tbsp oil to a large roasting tin or 2 medium tins. Put them in the oven to heat the oil for 10 mins.
Drain the potatoes and gently shake them in the colander to fluff up the sides a bit. Let the potatoes steam dry in the colander for a few mins.
Take the hot tin out of the oven and tip in the potatoes. Turn them to coat them in the hot oil. Return the tin to the oven and roast for 30 mins.
After 30 mins take the tin out of the oven and turn the potatoes. Scatter the fennel and cumin seeds over the spuds, then slide the tin back into the oven and roast for a further 15-20 mins till golden brown, turning the potatoes one more time halfway through. Serve straight away.