Sterilise a large jar by washing, then rinsing, in very hot soapy water. Then leave the jars to dry in the oven at 120 degrees. (Jamie doesn’t say you should do this; I always think, why not?)
150g long fresh red chillies.
Take the pan off the heat and allow it to cool. The liquid will become more syrupy, then from syrup to viscous and from viscous to jelly-like as it cools.
After about 40 minutes, or once the red flecks are more or less evenly dispersed in the jelly (as the liquid firms up, the hints of chilli and pepper start being suspended in it rather than floating on it), ladle into your jars. If you want to stir gently at this stage, it will do no harm. Then seal tightly.