Combine the mayonnaise, basil, lemon zest and a squeeze of juice, and some freshly ground black pepper in a small bowl.
Spread the mayonnaise mixture over all 6 slices of bread, then divide the smoked chicken between 3 slices and add some watercress. Top with the remaining 3 slices of bread to make 3 sandwiches.
To serve, remove the crusts, if you like, and cut into quarters.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.