Mix together flour, salt, and 50g of the caster sugar in mixing bowl. Rub in butter till mixture resembles breadcrumbs. Add egg yolk and mix (pastry will be crumbly). Press into a lined sandwich tin and top with baking parchment.
Bake pastry blind for 15 minutes in the preheated oven. Remove from oven and set aside.
Peel and chop apple place in a saucepan with water and golden syrup and cook until soft. Set aside.
Place egg whites in large bowl and beat using electric beaters for a minute, then gradually add caster sugar and beat until stiff.
Put apple mixture on base and pile meringue on top.
Place back in oven for about 20 minutes until meringue is nicely brown and set. Let pie set for awhile before cutting.
I didn't bake this blind when I tried it first and found the pastry a bit soggy, so blind baking is recommended.