Spicy prawn cocktail salad

Ingredients

• 1 small head of lettuce, finely shredded
• Large handful of watercress, shredded
• 260g king prawns, cooked and shelled
• 1 ripe mango, peeled and thinly chopped
• 1 avocado, peeled, stoned and roughly chopped
• For the sauce
• 4 tbsp mayonnaise
• 1 tbsp tomato ketchup
• Juice of 3 limes
• 1 medium red chilli, finely chopped (remove the seeds if you like it milder)