Seafood Cocktail with Langoustines and Marie-Rose Sauce

This is a bit of a bastardisation of another recipe I found in a
magazine with some added bits like the freshly-poached langoustines.

To poach the raw fresh langoustines (can be done a day in advance):
Boil a large pan of water with some fresh thyme, a little sea-salt,
chopped chives, and ripped parsley. Leave it boiling for about 10
minutes to infuse the water with the herbs, then throw in the raw
langoustines for about 3 minutes - any more and you will risk making
the meat too tough. Remove, place in a large bowl, and cover with
tinfoil. Put somewhere to cool, and then refrigerate to chill. DO
NOT BE TEMPTED TO RINSE THEM OVER.

For the Marie-Rose sauce (qty serves 4):
- 3 heaped tbsp mayonnaise
- 1 heaped tbsp ketchup
- Splash of Worcestershire sauce
- 1 tbsp good brandy
- Juice of 1 lemon
Simply mix it all together until it's loose and pink. A little
squidge of lemon juice gives it a good tang.

For the prawn cocktail itself:
- 1 langoustine per person (as you poached earlier)
- Tiger prawns
- Medium-sized prawns
- Cooked crayfish (I prefer the ones *not* in brine)
- Gem lettuce (1 per 4 servings)
- Cherry tomatoes
- Freshly sliced lemon and lime
- Cayenne pepper

Arrange the plate with 2 leaves of gem lettuce first. Put a teaspoon
of the sauce you made into each lettuce leaf, and arrange the mix
of prawns and crayfish to one side. Remove the claws from the
langoustine - we didn't use those this year (I made a stock instead)
but you may want to put them on the plate for decoration. Shake a
little cayenne pepper over the plate, then add the halved cherry
tomato and finally a slice lemon and lime for garnish.