Peel and cut the potatoes into large similar sized chunks. Place them in a large saucepan and cover with cold water and add a pinch of salt.
Bring the saucepan to the boil on a hot hob and cook for a further 15 mins until the potatoes can be pierced with a fork. Drain them into a colander in the sink and place back into the saucepan, cover the top of the saucepan with a clean tea towel and firm the lid back on. Allow to steam dry for 5-10 mins so the fluff up.
In the meantime wash, trim and slice the leeks into 1cm slices. Add a little olive oil to a pan on a medium heat and gently fry the leeks until just about softened. Drain them onto some kitchen paper.
Gently heat the milk and butter in a small saucepan. Mash the potatoes in the saucepan with some seasoning the pour in the warmed milk. Mash this thoroughly until lump free. Add in the leeks and mustard and stir rapidly with a wooden spoon until both well combined and you have fluffy creamy potatoes.