Put elderflower heads in a large pan with the lemon zest, juice, sugar and vinegar.
Add the water, cover with a clean tea towel and leave for 24 hours, stirring occasionally until all the sugar has dissolved.
Strain the liquid through muslin or kitchen paper and pour through a funnel into strong sterilised screw top or flip top bottles. Seal immediately and store in a cook dark place for at least 2 weeks. Please note that this will become fizzy so may pop the lid off if not secure. Can be kept for up to 1 year.