Heat the olive oil in a large saucepan over a medium heat and sweat together the garlic, carrot and potato for 5-7 minutes until soft, stirring occasionally. Stir in the diced tomato and fish stock and bring to the boil. Reduce to a simmer and simmer for 20 minutes.
Meanwhile, heat a large saucepan over a high heat for a minute then add the mussels and wine and cover. Let the steam cook the mussels for 3-4 minutes until they have opened. Strain the mussels and discard any that haven't opened. Remove the mussels from the shells and reserve in a bowl to one side.
After the soup has simmered for 20 minutes, add the octopus chunks and prawns and poach it in the soup for 8-10 minutes over a low heat. Adjust the seasoning as necessary then stir in the basil leaves and julienned basil. Add the mussels and frozen peas just before serving to warm through, then spoon into serving bowls and serve immediately.