Place the celery, carrot, onion, fennel, bay leaves and peppercorn in a large pan with the water and vinegar and bring to the boil. Add the octopus and cook for 30 minutes to 1 hour depending on the size of the octopus. The octopus is tender when you can easily slide a knife through the thickest part of its tentacles.
Remove the octopus from the liquid and cut it into small pieces approx 5mm long.
Combine the octopus in a bowl with the tomatoes, green olives, garlic, chilli, olive oil and thyme.
Note: Reserve the octopus juice and use it to make soup.