Heat the rhubarb pieces and apple cubes with preserving sugar, star anise, cinnamon and vanilla in a clean saucepan and bring to the boil briefly.
Add citric acid and sugar to taste and bring to the boil again briefly (do a gelling test!). Keep hot until bottling, stirring the jam again and again.
Use a stirring rod to carefully chop the rhubarb, the apple cubes should remain whole.
Fill clean jars to within 2 cm of the rim, if possible without dirtying the rim.
Briefly rinse the lids with brandy, drain and screw onto the hot jars. Cool slowly under a blanket, making sure all lids ‘pop’.
Apply labels to the jars.