Shake any obvious bugs off the elderflower heads then put them into a large jar or jug. Add the lemon peel, caster sugar and pour over the gin. Cover and leave to infuse for 24hrs.
Strain through coffee filter paper or 2 sheets of kitchen paper into a clean bottle. Keeps for up to 1 month. Serve with tonic water, lots of ice and a dash of sugar syrup if you like.