Bring a large pan of salted water to the boil, then add the langoustines and cook for one minute. Drain the langoustines and set aside to cool slightly before removing the tail meat. (Reserve the shells for another recipe, such as for making stock.)
Place the chicken stock into a pan and bring to the boil. Cook for 6-8 minutes over a high heat, or until the stock has reduced in volume by half. Stir in the cream and cook for a further 6-8 minutes, or until the liquid has reduced in volume by half again.
Stir in two-thirds of the butter a little at a time, until the butter is incorporated into the sauce, then whisk in the lemon juice and season, to taste, with salt and freshly ground black pepper. Keep warm.
Melt the remaining butter in a frying pan until foaming, then add the langoustine tail meat and fry for 2-3 minutes, or until golden-brown and heated all the way through. Remove from the heat and set aside.
In a large bowl, mix together the Little Gem lettuce leaves, salad leaves and herbs. Whisk the red wine vinegar and olive oil together in a bowl until well combined, then drizzle over the salad and herbs and lightly mix to coat evenly.
To serve, divide the salad among four serving plates, then place the langoustines alongside and drizzle over the sauce.