This recipe calls for a rectangular tart tin, but you can use a 23cm-diameter round one instead.
Note To test for a jam's setting point, place several saucers in the freezer before you start cooking the jam. When the mixture becomes thick, remove from heat, spoon a little onto a chilled saucer and return to freezer for 30 seconds. When you draw your finger through the mixture it should hold a trail. If it doesn't, cook for 1-2 minutes longer and test again.