Slice the cucumbers lengthways into about 5 mm-thick slices. Place them in a colander, sprinkle with the salt and mix gently.
Cover and leave to rest for 20 minutes to draw out excess moisture.
Meanwhile, place the vinegar, sugar and 200 ml water in a saucepan. Heat gently until the sugar has dissolved, but do not boil. Remove from the heat and allow to cool until lukewarm.
Lightly press the cucumbers to remove excess liquid, then rinse briefly under cold running water.
Layer the cucumber slices neatly on top of one another in a sterilised glass jar, packing them in quite tightly. Scatter over the coriander seeds, then add the bay leaf, garlic and chilli if using.
Pour the cooled pickling liquid over the cucumbers, ensuring they are completely submerged.
Seal the jar and leave to pickle for at least 24 hours before using.
Store in the fridge for up to 2 weeks.