Remove as many of the green parts, or cut the petals off with scissors. Add to a saucepan with the water and slowly bring to the boil. Let boil briefly, cover and let stand overnight.
The next day strain though a fine sieve or a colander lined with several layers of cheese cloth muslin. Pour back in the saucepan, add sugar and lemon juice and simmer at low heat to the desired consistency. Do not cook until thick, as the syrup will become much thicker when cold.
Fill in a sterilised bottle and close with a cork. Store in the fridge and use within 1 month.