Tip the courgette into a preserving pan with the sugar and slowly bring to a boil over a low heat.
Skim off any scum that comes to the surface and then tip in the lemon zest and juice and boil the mixture until it reads 104C on a cooking thermometer. Keep bubbling the mixture until a spoonful on a frozen plate wrinkles when pushed. Test every 5 mins.
Leave the mix to cool slightly then ladle into sterilised jars and keep somewhere cool and dark for up to one year. Use as you would any other lemony jam or instead of mango chutney.