Preheat the oven to 200°C/gas mark 6. Put the langoustines in a roasting tin and toss with the olive oil, half the lemon zest and all but 1 tbsp of the juice. Season with black pepper. Roast for 10-12 minutes until cooked through.
Combine the mayonnaise with the rest of the lemon zest and juice, the garlic and the chives or parsley. Serve with the langoustines and lemon.