Slice the lemon and place at the bottom of a large heatproof bowl. Don't worry about any pips that go in.
Gently shake the elderflowers to allow any insects to escape. Remove the flowers from the stalks and add to the lemons. A fork works well for this. Alternatively, place the heads face down in the bowl so that the stems are poking upwards.
Dissolve the sugar in the water over a low heat, stirring occasionally until it's dissolved. Bring to a rapid boil, then remove from the heat.
Stir in the citric acid, if using, or vinegar and stir. Pour the hot syrup over the elderflowers and lemon.
Cover the bowl with a plate or clean tea towel and leave the elderflowers to soak for twenty four to thirty six hours.
Line a large sieve with a clean muslin cloth and sit it over a large clean jug. Ladle the liquid into the sieve to filter out the flowers, lemon and any other unwanted bits.
As soon as the jug is full decant the liquid into sterilised bottles. Unless you have a steady hand, you might want to use a funnel to pour the liquid into the bottles. Continue to strain until it's all bottled.
Store in the fridge for up to three months, if using citric acid or six weeks if using vinegar.
