Add the first quantities of sugar and water into a heavy-based pot. Heat, stirring occasionally until it develops a caramel colour. Be careful not to overheat or it will become burnt.
Once the caramel colour has developed, slowly begin to add the second measurement of boiling water.
Once all the water is added, add in the second measurement of sugar. Add in a few slices of lemon now too.
Turn to a low heat and slowly simmer for 45 minutes. If it is still too runny and not thick enough keep going up to an hour. To test, put a few drops of the syrup on a plate, if it holds its shape and is thick when you run your finger through it, it should be ready.
Remove the lemon slices from the golden syrup.
Sterilize two-pint jars and make sure the jars are warm before ladling the cooled golden syrup into the jars. Let the jars stand until cooled enough to place lids on without causing condensation to build up on the lids.
Store in a cool pantry.
Notes
This recipe should keep for a few months in the pantry. It is worth mentioning if left too long, the sugar can sometimes crystalize.