Posted: 23/08/2015 at 18:43
Having had a day of diasterous jam making I'm not sure I should offer any ideas (!) but it does say in my book (by Thane Prince; the lady in charge of the jam section on the Allotment Challenge if you watched it; strong bob hair cut!) that you blitz a lemon shell ( which apparently is what you have left when you've juiced a lemon), then boil it in some water, and leave to cool. Strain. It produces a natural pectin. Use within 2 days. She doesn't say how much water to boil it in though.
Worth a go I guess, albeit Jo47 and others have provided better advice today than Ms Prince!