Measure the water into a small saucepan, sprinkle on the gelatine and set aside to soak for 5 minutes. Then place over low heat and stir until the gelatine has dissolved – do not allow the mixture to boil. Draw the pan off the heat.
Peel the bananas and cut into chunks. Place in a blender, along with the sugar and lemon juice. Cover and blend to a purée. Alternatively, the mixture may be well mashed together with a fork.
Whisk the chilled evaporated milk until thick. Whisk in the gelatine mixture and then fold in the banana purée. Pour into a glass bowl and chill until set firm. Serve with cream.