Put the cucumber in a large colander and sprinkle with ¼ tsp of salt. Leave to drain for 20 minutes.
Meanwhile, in a large bowl, add the remaining ingredients 1 – except the avocado, cucumber and mint – plus ¼ tsp of salt. Stir the ingredients well to combine.
Add the avocado and drained cucumber. Mix well, gently squashing the avocado into the mixture. Add the mint and mix again. This avocado tzatziki can be prepared a day ahead and eaten chilled with grilled pitta breads.