Heat oil in a heavy based sauté pan until very hot.
Sear meat on all sides until well browned. Place meat in slow cooker.
Pour red wine, Worcestershire sauce and water into the hot frying pan and simmer for a few minutes, scraping up all the browned bits stuck to the bottom of the pan. Transfer to the slow cooker.
Peel and cut the carrots into 2 inch lengths. Add to the slow cooker around the outside of the brisket.
Peel onions and, if small enough, leave whole. If not, quarter onions and place in slow cooker. Repeat with the garlic cloves.
Add thyme sprigs and bay leaf to the slow cooker, ensuring they are both submerged beneath the liquid.
Pop the lid on the slow cooker and set the temperature to low. Leave for 6-8 hours.
Transfer the slow cooked brisket and vegetables to a serving platter, cover and keep warm.
Pour liquid from the slow cooker (sieved if you prefer a smoother gravy - I leave mine with bits in it) into a medium sized saucepan and bring to a low simmer. Add sugar and stir until dissolved.
Dissolve cornflour in a little cold water (around 80 ml) and slowly pour into the simmering liquid, stirring until thickened.
Season well with salt and pepper.
Serve the slow cooked brisket and veg with mashed potatoes, gravy and Yorkshire puddings, if desired.