Slice the cooked potatoes into 5mm/¼in slices and place in a bowl with a drizzle of olive oil.
Gently cook the cream in a pan until reduced by half. Add the cabbage and stir until it wilts. Season. Season the langoustines with a little salt and olive oil.
Place a sauté pan on the heat, drizzle with a little olive oil and cook the langoustines on either side for 30-40 seconds or until golden. Add the butter and a squeeze of lemon jui ce, tos s the langoustines to coat, then remove with a slotted spoon and put on a warm plate to rest.
Toss the Jersey Royals into the same pan, add the chives and the remaining lemon juice and cook for a minute or two. Transfer to a bowl. Wipe the pan, return to the heat and cook the ham until crisp. Remove and set aside.
Arrange a dollop of the cabbage mix onto 4 warm serving plates and top each with 3 langoustines. Spoon the Jersey Royals and their cooking juice over and around, then finish off with a slice of crispy Parma ham and a sprinkling of mixed baby cresses.