What alternative gelling agents can I use for jam and marmalade?
Tapioca Jelly is interesting stuff: 


  A typical recipe for tapioca jelly can be made by washing 2 tablespoonfuls of tapioca, pouring a pint of water over it, and soaking for three hours. It is then placed over low heat and simmered until quite clear. If too thick, a little boiling water can be added. It can be sweetened with white sugar, flavored with coconut milk or a little wine, and eaten alone or with cream.


Of course, not all tapioca jelly recipes are yummy at first go, so you'll have to experiment.

Powdered tapioca starch is the starting material of choice, not the pearls or cubes etc.