Preheat the oven to 190°C/170°C fan/gas mark 5. Put the potatoes in a small saucepan and cover with cold water. Bring up to the boil, then partially cover and simmer for 15-20 minutes, until tender. Drain well, then mash thoroughly. Return the potatoes to the pan, add the milk and butter and stir until hot and smooth. Set aside and keep warm.
Place the chipolatas on a baking tray and cook in the oven for about 25 minutes, turning them once or twice, until nicely browned.
Slice each sausage open once out the oven (but not all the way through) and pipe the mash in the centre. Serve with the cranberry sauce.