Shake the elderflowers to get rid of any insects hiding inside, then put them in a pan and cover them with water. Add the lemon zest and simmer for 30 minutes. Let it cool then strain it into a large bowl through muslin or a clean tea towel, gently squeezing the elderflowers to extract all the juice.
You should get about 1-1.2 litres of juice. Pour it back into the pan, then add the sugar and the juice of the zested lemon. Bring to the boil, then gentle simmer until it reduces by half into a thick cordial. Taste - if it’s not sweet enough for you, add some acacia honey.
Pour the liquid into clean, sterilised bottles and seal. Keep chilled in the fridge and it should last for several weeks.