Grease a 25cm flan tin. Roll out the pastry on slightly floured board, lift it carefully into the tin and press gently into shape. Trim the excess then press the pastry up around the edge again so it stands proud of the tin to allow for shrinkage. Prick all over with a fork. Chill for 30 minutes.
Peel and core the apples and slice thinly. Reserve the neatest ones to decorate the tart, pop them in a bowl with 25g of the caster sugar and the lemon juice and toss to coat. (This will stop them going brown)
Put the remaining apples in a saucepan with the water, cover and cook until soft.
Add the rest of the sugar, the vanilla, butter/margarine, cider and apricot conserve to the cooked apples and cook, uncovered and stirring frequently until you have a thick purée.
Preheat the oven to 200C/180 Fan, line the chilled pastry case with baking parchment and fill with baking beans. Bake in the preheated oven for 10 minutes. Remove the parchment and beans and bake for a further 5 minutes or until the pastry is dry and beginning to colour slightly.
Spread the apple and cider mixture evenly in the pastry case. Arrange the reserved apple slices prettily on top and bake at 180C for 30 minutes or until the apple slices are beginning to caramelise.
Serve hot, warm or cold with cream, custard or ice cream.