Belvoir is a favourite, but if doing some outdoor DIY here is a simple recipe. Tartaric acid is widely available in supermarkets and chemists and will not get you arrested if you buy it in large amounts...
Makes roughly 2 x 500ml bottles
Snap off any particularly thick stalks connected to the flower heads, leaving just the slim stems holding each blossom
Put the flower heads in a large preserving pan or bowl
Sprinkle over the sugar and tartaric acid
Peel the lemon rind & add to the pan, along with all the lemon juice
Add the hot water and stir gently but well. Cover loosely with a tea towel and leave to stand in a cool place for 24 hours, stirring every now and then.
The sugar should completely dissolve and the syrupy cordial will become infused with the flavours of elderflowers. Strain the contents of the pan through a muslin-lined colander into a clean bowl. Decant into sterilised bottles
Seal and store in sterilised bottles and store
Preheat the bottles in an oven at 180c/350f/Gas 4. Wash the bottles & stoppers really well and put on a baking sheet in the oven for 10 minutes. Leave to cool slightly before adding the cordial through a funnel.