Put the potatoes in a pan of cold water, bring to the boil and simmer for 5 mins. Meanwhile, put the butter and oil into a large, shallow roasting tray and heat in the oven.
Drain the potatoes in a colander, giving them a good shake to fluff the edges up a little. Toss in the hot oil in the tray and roast for 20 mins.
Add the garlic, rosemary and chipolatas to the tray, toss well together and roast for a further 50 mins until the potatoes are crunchy and golden brown and the sausages are cooked through.
