Mix the crab, mayo, lemon zest, a little juice, the chives and a good grind of black pepper in a bowl. Butter one side of each
slice of bread.
Divide the crab mayo between 3 slices of bread, top each with another slice, butter-side up, then arrange the cucumber slices on top, season, and finally sandwich with the final pieces of bread, butter-side down. Cut off the crusts, then cut into little triangles.