Parboil the potatoes in a large pan with plenty of water and 1 teaspoon of salt.
Cook them until you can just stick a fork or knife into them with some resistance (about 10 minutes but this varies depending on the size) then remove from the heat and drain.
Leave to cool (or until just warm but not hot) then put the potatoes carefully in the freezer bag or lidded container.
Pour in the oil and the rosemary then seal the bag/container and give it a good shake around to distribute the oil and herbs all over the potatoes.
Leave to marinade in the fridge for as long as you can but at least 30 minutes. They can be prepared like this up to 24 hours in advance.
When you are ready to cook the potatoes, remove from the fridge and thread onto skewers. I have metal skewers that I use but if you use wooden skewers make sure you soak them in cold water for a while first so they don't go up in flames.
Place on the grill and cook for around 20 - 30 minutes, turning frequently.
Once golden brown, very soft and slightly charred, remove from the grill and serve. Be very careful as they will be exceptionally hot inside.