Roughly cut up the apples then place into a preserving pan, pips, core and all. Add enough cold water to cover. Bring to the boil, reduce the heat and allow to simmer for 30-40 minutes, or until the fruit is pulpy.
Pour the fruit and liquid into a sterlised jelly bag and allow to drain through into a large container until it stops dripping, about 3-4 hours.
Measure the juice and allow 450g/1lb preserving sugar and 5g/1 tsp of chilli flakes for every 500ml/17fl oz of juice. Place the juice, sugar and chilli flakes in a clean pan. Bring to the boil, stirring until the sugar has dissolved. Reduce the heat and carefully skim off any scum from the surface. Return to the boil and boil vigorously for 15-20 minutes or until setting point is reached, skimming regularly. Using a sugar thermometer, the setting point will be reached at 105C/220F.
Remove the pan from the heat and leave to settle for a few minutes. Skim the surface once again. Pour the liquid jelly through a wide mouthed funnel into jars that have been sterilized (put in oven at 90C/200F/Gas¼). Seal each jar with a lid. This jelly will keep for 2 years, unopened.
