In a food processor, blitz together flour, sugar and salt. Add butter and PULSE a few seconds at a time until the butter assembles the size of peas.
Combine milk and egg yolk in a little bowl. Add this to the food processor and pulse, until a ball begins to form. Tip the mixture out and shape into a disc with your hand.
On a floured worktop, roll out the pastry to line the greased and floured tin. Press firmly against the tin to make sure no gaps are there. Chill for half an hour or freeze for 15 minutes.
Preheat oven to gas mark 6 / 200'C / 400'F.
Once the custard filling is cooled, pour into the crust. Cut the excess dough to 1/2 cm of the custard level.
Bake for 45 mins until slightly wobbly.
Put under the grill for 3 - 5 minutes until it's slightly burnt on top.
Leave to cool then put in the fridge to cool for at least 4 hours.