Preheat the slow cooker if necessary; see the manufacturer's instructions. Lightly butter 4 metal individual pudding moulds, each 250 ml (8 fl oz). Pour the sugar and water into a small saucepan and heat gently, stirring occasionally until the sugar has completely dissolved.
Increase the heat and boil the syrup for 5 minutes, without stirring, until the syrup has turned golden, keeping a watchful eye as it cooks. Take the pan off the heat, add the boiling water and stand well back. Tilt the pan to mix, and when bubbles have subsided pour into the pudding moulds, tilting them so that the syrup coats the base and sides.
Put the eggs and egg yolks into a bowl and fork together. Pour the condensed milk and fresh milk into a saucepan, bring to the boil, then gradually beat into the egg mixture until smooth. Strain back into the pan, then stir in the lemon rind.
Pour the custard into the syrup-lined pudding moulds, then transfer the moulds into the slow cooker pot. Cover the top of each one with a square of foil. Pour hot water around the moulds so that the water comes halfway up the sides, then cover with the lid and cook on low for 2½–3½ hours or until the custard is set with just a slight wobble in the centre. Lift out of the slow cooker pot with a tea towel, cool then transfer to the refrigerator for 3–4 hours or overnight to chill.
Dip the base of the moulds into boiling water for 10 seconds, loosen the top of the custard with a fingertip, then turn out on to rimmed plates.