Put the sugar and 1.5 litres/ 2 3/4 pints water into a large sauce pan. Heat gently, without boiling, until the sugar has dissolved. Give it a stir every now and again.
Pare the zest from the lemons using a potato peeler, then slice the lemons into rounds.
Once the sugar has dissolved, bring the pan of syrup to the boil, then turn off the heat.
Prep the flowers. Fill washing up bowl with cold water. Give the flowers a gentle swish around to loosen and dirt or bugs.
Lift flowers out, gently shake and transfer to syrup along with lemons, zest, and citric acid, then stir well. Cover pan and leave to infuse for 24 hours.
Strain cordial - Line a colander with a clean tea towel, then sit it over a large bowl or pan. Ladle in the syrup - let it drip slowly through. Discard the bits left in the towel.
Prep and fill bottles. Use a funnel and a ladle to to fill and sterilize bottles.
The cordial is ready to drink straight away and will keep for 6 weeks in the fridge.