Preheat the oven to 200°C/gas mark 6. Boil the potatoes for about 10 minutes, until tender. Drain the potatoes and leave to steam dry in a colander for a few minutes, then mash the potatoes with the butter and milk to create a smooth mash.
Heat the olive oil in a pan and sauté the chicken thigh fillets and leeks for 6-8 minutes until the chicken is cooked through. Transfer to a large ovenproof dish and add the frozen peas.
Make up the instant cheese sauce mix according to instructions on the packet and add to the dish. Mix well. Spoon the mashed potato on top, spread evenly and mark with a fork, then bake for 30-35 minutes until golden and bubbling.