Cut the cucumbers in half lengthways, then cut each half into spears.
Put the white wine vinegar and water into a saucepan. Add the garlic, sugar, salt, chilli flakes, fennel seeds, cumin seeds and mustard seeds. Bring to the boil, stirring until the sugar and salt have dissolved, then remove from the heat. Leave the brine to cool completely.
Pack the cucumber spears into a large clean jar, such as a jar previously used for gherkins.
Pour the cooled vinegar brine over the cucumbers, making sure they are well covered.
Seal the jar and refrigerate. Leave the cucumbers to pickle for at least 2 days before opening, although they will develop even more flavour if left a little longer.
Keep refrigerated and use within a reasonable period once opened