Place eggs in a large saucepan, cover with cold water and bring to the boil over high heat. Cook for 7 minutes, drain and cool under cold running water. Peel and mash using a fork, stir through curry powder and mayonnaise and season to taste with sea salt and freshly ground black pepper.
Butter bread and spread egg over half the slices, top with watercress and remaining bread. Using an electric or serrated knife, remove crusts, cut sandwiches into quarters and serve immediately or cover with a damp tea towel and refrigerate until required.