Pre-heat the oven to 220°C, 425°F, Gas Mark 7.
Place the tuna in a bowl and add celery, red pepper, mayonnaise, half the beaten egg, Lemon Pepper and Chives, mix carefully.
Roll out pastry to an oblong measuring approx 35.5 x 30.5cm (14 x 12"). Transfer to a baking tray.
Spread the filling lengthwise down the centre third of the pastry. Cut horizontal, 2.5cm (1") width strips down each side of the pastry to within 2.5cm (1") of the filling. Fold the end pieces of pastry over filling and lift strips of pastry over the filling from alternate sides to form a plaited effect. Brush with remaining beaten egg. Cook in the oven for 25 minutes until golden.
Serve hot or cold garnished with green vegetables or salad stuffs as applicable.