Buy Maris Piper or King Edward. They are widely available, and will suit roasties and mash. Don’t use ‘waxy’ potatoes, like Charlotte.
Allow 1-2 potatoes per person.. I always add a few more in too. They never go to waste. Peel and chop into a large dice. Rinse in plenty of cold running water. Bring to a simmer, over a medium high heat for roughly 30 minutes. Insert a sharp knife. If the potato is cooked, drain into a colander or sieve. Make sure that the potatoes are dry. Mash the potato by hand or use a ricer, while it is still warm. Do not use an electric whisk or food processor. This can make something reminiscent of wallpaper paste. It’s very trendy for Michelin starred chefs to add equal amounts of potato and butter, and sieve it. I mash the potatoes, and add a generous amount of butter, some milk and then I add some more butter.