Peaches: Place the peaches in the bottom of an ovenproof baking dish.
Custard: Measure the juice from the peach tin and add milk up to 400ml.
Melt margarine in a small saucepan over a medium heat until melted then add the flour and stir until its all combined.
Remove the saucepan from heat then add the milk and peach juice a bit at a time, beating well until all combined then return the saucepan to heat. Stir regularly so that it doesn't curdle.
Place egg whites in a bowl then add yolks to the heating saucepan and mix well. Once sauce has thickened place over peach slices and leave to cool.
Meringue: Whisk the egg whites until nice and fluffy. To ensure it is ready place bowl upside down if meringue stays stuck at the bottom of the bowl and doesn't move it's ready to continue with the next step if not carry on whisking.
Add caster sugar gradually whilst whisking until all combined. The meringue should be lovely and white. Pour over the cooled custard.
To Cook: Place in the oven and cook for about 15 minutes or until golden brown. Leave to cool then serve.