Prepare the rhubarb by washing, trimming the ends and any bad bits and cut into roughly 2 cm pieces.
Place rhubarb, sugar, lemon juice, gin, vanilla seeds and the lemon skin into a bowl and leave to soak for an hour or overnight.
Meanwhile sterilise 2 x 250 ml jars (see notes below) and put a couple of saucers in the freezer for testing the set.
Start to make the jam by tipping the rhubarb mixture into a preserving pan or similar.
Place pan on a gentle heat to allow the sugar to completely dissolve, stirring the whole time (do not simmer).
Once the sugar has dissolved (any gritty noises will have vanished) turn up the heat and bring to a rolling boil and then start timing for 8 minutes
Take the pan off the heat and place a few drops of jam onto one of the chilled saucers then pop in the fridge for a minute.
Then push your finger through the jam and if a gel has formed then the jam is ready, if the jam is still liquid then boil for another 2 minutes and so on until set is reached.
Remove the half lemon skin then pot up into the sterilised hot jars and seal with lids immediately.
Store in a cool dry place. Once open store in fridge and use within 2 - 3 months.