Pat the prawns and crevettes dry on kitchen paper.
Mix the mayonnaise, ketchup, Worcestershire sauce, lemon juice, Tabasco, brandy or sherry (if using) together then season.
Cut 3 of the gem hearts into thin wedges and shred the remaining one. Halve the cucumber lengthways and remove the seeds with a teaspoon, then cut into long diagonal slices. Remove the skin and stone from the avocado and slice the flesh.
Scatter the lettuce, cucumber and avocado over a large platter. Top with both the peeled prawns and drizzle with half the dressing.
Dot with spoonfuls of the salmon caviar (if using), crevettes and freshly ground black pepper. Serve with remaining dressing, lemon wedges and buttered wholemeal bread.