Bring the water, sugar, lemons and vanilla pod to the boil. Simmer for a few minutes and leave to cool.
Strain the syrup.
Put the banana flesh into a processor and whizz. Add to the syrup and mix well together. You want a density of 22° on the saccharometer.
Place into the ice cream machine or deep freeze. If using the deep freeze, bring out from time to time and stir.</