The best ham hock, leek & potato gratin

Potato, milk, beechwood smoked ham hock (15%) (pork (92%), water, salt, preservative (sodium nitrite)), leek (8%), water, mature cheddar cheese (milk), double cream (milk), rapeseed oil, cornflour, wheat flour (wheat flour, calcium carbonate, iron, niacin, thiamin), butter (milk), salt, black mustard seed, spices, spirit vinegar, parsley, yeast, olive oil.