Combine everything in an stainless steel pot, put on the fire and stir until it thickens. 85 C or possibly thereabouts.
Do not use bottled juice, it tastes awful.
Zest of lemons, finely chopped. Unsprayed lemons! Whatever it takes to make 50g.
A quality lemon compound from Doehler, Hero, etc. Has more flavor and is guaranteed chemical free.