Steam the baby new potatoes until tender (this takes 20-30 minutes). When cooked, turn off the heat, pour off the water from the pan below the steam pan, then sit the perforated pan on top of the empty, hot pan with the lid off to dry the potatoes. If it makes your life easier, you can steam the potatoes a couple of hours in advance as it would be fine roasting them once they're cooled, so long as they haven't been in the fridge.Preheat the oven to 220ºC/200ºC Fan/425ºF then pour the oil into a small, shallow roasting tin and heat it in the oven for 5 minutes.Tip the potatoes out onto a plate, and crush with a fork, but not too throughly. You want some of them crumbling into small pieces, but mostly think of more-or-less halving, leaving rough edges, the better to crisp and brown in the oven.Take the tin out of the oven, carefully turn the potatoes in the oil and then roast for 20 minutes, then turn them and cook for a further 10 minutes until they are deep golden brown in parts and the rough edges are crisp; the smaller pieces will be dark and crunchy.Remove to a serving bowl and sprinkle with vinegar and salt, then taste one of the potatoes — ultimate sacrifice — to see if you need to add more of either.