In a large bowl combine the olive oil, thyme, salt and pepper. Place the sardines in the bowl and toss to coat. Place on a baking tray lined with baking paper and set aside.
In the same bowl toss potatoes in oil with thyme a generous sprinkling of salt and pepper. Place potatoes cut side down in a single layer on a baking tray.
Set tray on lowest rack of cold oven. Heat oven to 230 degrees C. Roast until golden brown and potatoes are tender; 20 to 25 minutes. Stir halfway through cooking to brown evenly.
After the potatoes are cooked set them aside to cool slightly. Heat the grill and place the tray with the sardines under the grill. Grill for approx 5 minutes per side, depending on the size of the sardines, until they flake easily with a fork.
Meanwhile, transfer potatoes to a large bowl; add olives, shallot and parsley.
Whisk lemon juice, mayonnaise, garlic, sea salt and pepper in a measuring jug. Slowly whisk in oil in a slow steady stream. Pour dressing over potatoes; toss to coat.
Dish potatoes onto six plates and place two grilled sardines on top each.