Put 800ml/27fl oz of the milk in a saucepan with the sugar, teabags, vanilla pod, cinnamon, cardamom pods and allspice and bring to the boil. Reduce the heat and simmer gently for 5 minutes then turn off the heat, cover and leave for 10 minutes. Discard the teabags and strain into ice cube trays. Freeze.
Place 4 tall glasses in then freezer.
To serve, put the ice cubes in a blender with the remaining milk and the ice cream then blend until smooth and pour into the 4 glasses.