Preheat your oven to 180C. On a baking sheet place your jars and bake them for 10 minutes. Remove carefully (very hot!).
Clean and trim the rhubarb into 2cm pieces. Peel and halve the bananas. Combine rhubarb, banana, sugar, pectin and lemon juice in a large, heavy based pan and leave to stand for 2 hours. Stir occasionally.
Turn on the heat, bring to the boil and boil for 4 minutes. Check setting consistency of the jam on a cold plate.
If desired you can smooth the jam with a hand blender. Through a funnel, pour the hot jam into the jars. Seal the lids and (using oven gloves!) turn the jars upside down. Leave to cool