Use the pastry sheet to line a shallow Grundy tin, and crimp edges if desired.
Scatter the drained leeks over the base of the pastry, then top with tuna and sweetcorn.
Beat together the eggs, milk and seasonings and carefully pour into the pastry lined Grundy tin. Bake for approx. 30 minutes until 'custard' is set and pastry edges golden. Serve warm or cold with vegetables or salad.