Place the apricot halves, lemon juice and water in a large non-reactive saucepan and bring slowly to the boil, stirring occasionally. Reduce the heat and simmer for 15 minutes, or until the fruit is soft.
Remove from heat, add the sugar and stir until it has dissolved completely. Return the pan to the heat, add the butter and stir until melted.
Bring the jam to a full boil and boil rapidly for 15 minutes, then test for setting. If setting point has not been reached, boil for 5 minutes more, then test again. Pour into warm sterilised jars and cover immediately.
How to test jam for setting
Test for setting by spooning a small amount onto a chilled saucer. Chill for 3 minutes, the push the jam with your finger; if wrinkles form on the surface, it is ready.