For the crab: Check through the crab meat and discard any piece of shell. Squeeze out any liquid. Mix the crab meat with 1 tablespoon of the mayonnaise, ½ tablespoon lemon juice and the dill leaves. Season with a little salt and black pepper. Taste and adjust the seasoning as necessary.
For the basil mayonnaise: Add the rest of the mayonnaise, the garlic, lemon juice and the basil leaves to a mini food processor. Whiz for 1-2 minutes until the basil is finely chopped and mayonnaise turns a lovely green colour. Season with a little salt and black pepper. Taste.
To serve: Divide the watercress or mixed leaves between four serving plates. Place a spoonful of the crab mixture in the middle and surround it with the prawns and smoked salmon. Drizzle over the basil mayonnaise. Serve any remaining mayonnaise on the side.