Preheat the oven to 180°C (360°F) or 160°C (320°F) for fan-forced ovens. Place a 20 or 23cm (8 or 9in) round cake ring tin into a deep baking tin. To make things easy, place a large heatproof sieve and mittens near the tins ready for use. Fill in your kettle with water up to the maximum level, then boil it.
In a blender, place in the eggs, then pour in the milk, evaporated milk, condensed milk and vanilla extract. Blend on high for 30 seconds or until there are no egg bits still visible. Leave the mixture in the blender jug and set it aside whilst working on the caramel.
In a small saucepan, place in the sugar and water, then place it over medium heat. Once the mixture bubbles, put a lid on the saucepan and cover for a minute (doing this dissolves any sugar crystals on the sides of the pan). Uncover and reduce the heat to medium-low, leaving sugar undisturbed (don't mix). The mixture will start darkening and go from clear to amber and from here it can easily burn and if it does, you'll have to start over again. As soon as the mixture develops a golden amber colour, remove the saucepan from the heat.
Working quickly but carefully, pour the caramel into the cake ring. Using mittens, tilt the cake tin so the caramel coats onto the sides of the tin and onto the middle. Coating the caramel around the tin will help with the removal of the flan. Pour the egg and milk mixture over a sieve and into the cake ring over the caramel. If you see any bubbles, pop them with a fork. Pour the boiled water into the baking tin until the water reaches halfway against the cake tin. Bake the flan for 1 hour or until a skewer comes out clean when inserted in the middle.
Leave the flan to cool completely in its cake ring and leave it in the water bath. Once the flan has cooled down, place cling wrap/plastic film over the flan and leave it in its tin. Chill in the fridge for at least 4 hours or overnight. Before serving, run a small flat knife around the edges of the tin and around the outside of the inner ring. Place a large plate over the tin and turn the tin with the plate, the flan should fall onto the plate.