Preheat the oven to 220 degrees centigrade. Cook the potatoes in a large pan of boiling water for 10 minutes until almost tender and starting to crumble slightly around the edges.
Drain the potatoes well and transfer to a large roasting tin. Break the garlic bulb and nestle the cloves between the potatoes. Roughly chop the tomatoes and scatter over the top with the rosemary. Season generously, then drizzle over 2 Tbs of olive oil. Roast for 15 minutes.
Add the cod fillets to the pan, allowing them to rest on top of the potatoes and tomatoes. Drizzle the remaining oil over the top, season the fish and cook for 5 minutes. Splash in the wine and cook for a further 5 minutes until the fish is cooked.
Tear over the basil leaves and serve straight from the pan. Remember to remind your guests that they'll need to peel the cloves of garlic before they eat them.