Rinse the potatoes and roll them lightly in sea salt. Bake them in an oven preheated to 200 C Gas 6 until knife tender. This will take about 40 minutes.
Meanwhile drizzle the tomatoes lightly with olive oil and a little salt, then roast for 30 minutes in a shallow tin. They are ready when they start to burst their skins.
Crush the potatoes lightly with a heavy spoon or potato masher so they crack open.Spoon the tomatoes over and to the side, leaving their juices behind in the tin.
Stir the balsamic vinegar into the juices with most of the black pepper, putting the rest of it into a little dish for those who want more.