1.Place eggs in a small saucepan; cover with cold water, cover pan with a lid. Bring water to the boil, then remove lid. Boil for 5 minutes, then remove from heat; drain. When cool enough to handle, shell eggs.
2.Roughly mash eggs in a medium bowl with yoghurt, mayonnaise, mustard, oil, capers and celery.
3.Combine beetroot and radish in a small bowl.
4.Top each bread slice with half the watercress, beetroot and radish mixture, then egg mixture. Sprinkle with chives; season with pepper to taste.
You can make the egg mixture a day ahead. Store in an airtight container in the fridge and assemble sandwiches just before serving.Note