Place a saucer in the freezer.
Combine all ingredients in a heavy based saucepan over a medium heat.
Bring to the boil slowly, stirring constantly, until the sugar is all dissolved.
Cook stirring occasionally to prevent sticking.
After about 15-20 minutes start testing to see if it's set. Place a teaspoon of jam on the cold saucer. Set aside for a few minutes, then gently push it with your finger - if the surface wrinkles, the jam is set. If not, keep cooking for another 5-10 minutes before testing again.
Cool for 10 minutes, then carefully pour jam into hot, sterilised jars and seal. Cool completely before storing.