Top and tail the gooseberries, discarding the hard ends, and place in a large saucepan. If you’re using elderflowers, tie them up in a piece of muslin and secure with string, then add to the saucepan. If using cordial, add to the pan with 150ml water. Bring to the boil, then reduce the heat and simmer for about 30 minutes, or until the fruit is tender and has broken down nicely. Remove from the heat, squeeze any liquid out of the elderflower parcel and discard.
Split the vanilla pod lengthways and scrape out the seeds. Add the sugar and the vanilla pod and seeds to the pan and boil over a high heat for about 10 minutes, skimming off any foam that forms on the surface. Pour through a strainer into sterilised jars and leave to cool before putting sealing with the lids.