Preheat oven to 210c/410f.
Bring a large pot of water to boil, then add bicarb soda - it will fizz a little.
While the water is coming to the boil, peel your potatoes.
Carefully add the potatoes, and boil for 20 mins or until very soft.
When the potatoes have about 10 minutes to go, pour the oil into a metal roasting tin, and place in the oven to heat.
When potatoes are soft, drain well then return to the pot. Give them a mix around in the pot to rough them up, if they break apart a bit that's fine too.
Carefully add potatoes to now-heated oil, and turn well to coat them completely.
Bake for 1 - 1 1/4 hours, tossing again about every 15 mins (add rosemary and garlic cloves with approx 15 - 30 minutes to go if using).
Discard rosemary and garlic, add salt to taste and serve immediately.