Put the eggs in a saucepan of cold water and bring to the boil. Reduce the heat and simmer for 7-8 minutes.
Drain, cool under cold water, then peel and roughly chop.
Meanwhile, mix the flaked tuna together with the sweetcorn, mayo, white wine vinegar, spring onion, and salt and pepper (if using). Add the eggs and salad cress, stirring well.
Cut the pitta breads in half to give 4 pitta pockets, toast the bread and divide the mixture between them. As with other oily fish, tuna can contain environmental pollutants so stick to no more than two portions a week