Put the cranberries in a large saucepan and add enough water to cover them completely. Cook over a high heat for about 15 minutes until they soften. Then pass everything through a sieve (in a new pot).
Add the citric acid, preserving aid and preserving sugar and cook for another 3 minutes. In the meantime, peel the bananas and place in a tall container (e.g. measuring cup) and puree with the hand blender.
Put the banana puree in the saucepan with the cranberries and stir everything vigorously. Do not cook afterwards. Pour into prepared glasses.