Layer the cucumbers in a bowl with the salt, cover with a plate, weight down and leave to soak for 4 hours. Tip into a colander, drain off the liquid, then rinse with cold water and drain well.
Heat the oil in a preserving pan, add the onions and fry for 5 minutes, stirring until softened. Add the red peppers and fry for a further 5 minutes.
Add the vinegar and sugar to the pan. Mix the chillies, turmeric, mustard powder and cornflour in a bowl, then stir in the measurement water and mix until smooth. Stir into the vinegar mixture and mix until smooth. Cook gently for 10 minutes, stirring from time to time, until thickened. Stir in the cucumber and peppercorns and cook for 5 minutes.
Ladle into warm, dry jars, pressing down well and making sure that the vinegar mixture covers the vegetables. Disperse any air pockets with a skewer or small knife and cover with screw-top lids. Label and leave to mature in a cool, dark place for at least 3 weeks.