Preheat the slow cooker if necessary; see the manufacturer's instructions. Take the tentacles out of the squid tubes and rinse inside the tubes with cold water. Put them in a sieve and rinse the outside of the tubes and the tentacles. Drain well, put the tentacles in a small bowl, cover and return to the refrigerator. Thickly slice the squid tubes.
Heat the oil in a large frying pan, add the onion and fry, stirring, for 5 minutes or until golden. Add the garlic and cook for 2 minutes. Stir in the tomatoes, stock, capers, olives, thyme, fennel seeds, sugar and salt and pepper and bring to the boil.
Pour the sauce into the slow cooker pot, add the sliced squid and press the pieces below the surface of the sauce. Cover and cook on low for 3–4 hours.
Stir the squid mixture and add the tentacles, pressing them below the surface of the sauce. Cook on low for 30 minutes. Serve tossed with linguine and garnished with extra thyme leaves, if liked.