Place the caster sugar for the caramel in a small heavy-based pan and warm over a gentle heat, allowing it to melt. Once the sugar has melted, increase the heat a little to get a good caramel colour, then pour either into individual ovenproof dishes or into one large dish, coating the bottom completely. Leave to cool.
Heat the oven to 150°C/gas 2. Heat the milk to a shivering simmer. Beat the eggs and sugar together in a bowl, then add the hot milk and mix well. Cut the vanilla pod in half lengthways, scrape out the seeds and add them to the milk. Pour the mixture into the prepared dishes (if using individual pots, make sure to give the mixture a stir each time to distribute the vanilla seeds evenly). Place the dishes in a deep roasting pan and almost fill with hot water from the kettle. Place in the oven and cook for 45 minutes until set to the touch.
Remove and allow to cool, then turn out. Spoon any remaining caramel from the pots over the puddings, and serve.