Mix to combine all custard ingredients in a large pot, and simmer (stirring constantly) over a medium stove till thickened to custard consistency. Remove and cool slightly.
In the meantime, make syrup by combining water, sugar and lemon peel in a medium pan and simmer till reduced by half. Remove and cool until ready to pour over pie later.
Once custard is slightly cooled, arrange half the filo sheets (usually 10 or 12) in a large deep rectangular baking dish (around 35 x25cm - the rough size of the sheets), brushing with melted butter between each .
Pour custard filling and spread evenly, then repeat this with remaining filo sheets. Once you get to the last sheet, brush over the top with butter again, and this time with a sharp knife cut out about 12-15 servings on the filo, making sure the knife only cuts into the top 5 or 6 sheets of filo.
Bake till golden brown and remove from oven. Pour warm syrup over the warm custard pie. Serve no earlier than 30 minutes, so that pie has had a chance to absorb the syrup properly.