Clean the flowers by removing the stems and leaves, then shake the insects off the flowers.
Put the flowers into a large bowl and pour lukewarm water over them. The water should completely cover the flowers so add more, if it doesn’t. Put the sliced lemons on top, cover and rest in a cool place up to 3 days - at least for 24 hours.
Strain the blossoms and lemon through a sieve into another bowl, pushing all the liquid out of the soaked blossom and lemon slices. You should get about 1 -1.5 litre liquid. Pour this into a saucepan through a sieve, lined with a cheesecloth.
The liquid should be clear and golden. Add the sugar and bring to the boil. Simmer over medium heat until you get a thick, syrupy sauce (it should reduce to about a third). (If it's not thickening/reducing enough after 25 minutes or so, add some more sugar!) The thickness should be similar to golden syrup.
Pour the syrup into a sterilised jar and keep it in a cool place. You should get about 200-300ml.