Start by making pastry. Put flour, cheese, butter and salt in food processor and blitz for 1-2mins, or until mixture resembles fine breadcrumbs. Add eggs and milk and blitz for 2mins more, or until smooth dough forms. Wrap in clingfilm and refrigerate for at least 30mins.
Heat oil in large non-stick frying pan over medium heat. Add chicken and cook in batched for 4-5mins or until golden (adding more oil between batches if nec). Set chicken aside and keep warm. Add butter, leeks, garlic and thyme to pan and cook for 7-8mins, or until softened. Add flour and cook for 2mins. Add milk, season and cook for 3mins more, or until thickened. Return chicken to sauce, remove from heat and cool to room temp before refrigerating to cool completely.
Preheat oven to 180C. Roll half pastry out between 2 sheets of baking parchment (or clingfilm) to 0.3cm-thick circle. Line 28-31cm round, lightly greased pie tin with pastry and fill with cooled chicken mixture. Roll remaining pastry out between 2 sheets of baking parchment to 0.3cm-thick circle; place on top of pie and trim any excess pastry.
For egg wash, put egg and cream in bowl and mix to combine. Brush pastry with egg wash and bake for 35-40mins or until pastry is golden.