You could do this in a frying pan but using the oven makes it all the more easy.
Heat your oven to 200°C and line a tray with foil to make the clean up easier, spread a little oil over the foil to help prevent the sausages sticking. Get your bangers in the oven then peel the potatoes and get them on to boil.
When the potatoes are ready, mash them up with butter, milk, salt and pepper and mix in a half tablespoon of dijon mustard, or whatever mustard you have really.