1. Heat the lemon juice and sugar in a wide heavy based pan over a gentle heat, stirring until the sugar has dissolved
2. Add the lemon zest, cloves and marrow and bring the mixture to a rapid boil.
3. Cook for 8 – 10 minutes, and skim off any foaming scum, stir occasionally to ensure that the marrow is not sticking.
4. Reduce the heat to medium – low and simmer for around 20 minutes
5. The marrow should become translucent
6. Test the marmalade at this point, to see if it wrinkles as it cools, if so, the marmalade will set, if not, continue boiling
7. Pour the marmalade into sterilised jars and seal
8. Serve, spread on toast