Put the flour and oats in a bowl, then rub in the margarine or butter until the mixture resembles breadcrumbs. Add just enough cold water to mix to a soft but not sticky dough.
To make an oat pastry case preheat the oven to 200ºC/ 400ºF/ Gas 6 and lightly oil a 20cm/ 8 in flan ring. Roll out the pastry on a lightly floured surface, then use to line the flan ring. Cover with greaseproof paper and fill with baking beans.
If you are using the flan case for a filling which has to be cooked, bake in the oven for 10 minutes, then remove the paper and beans, fill with the chosen filling and return to the oven for about 20 minutes until both filling and pastry are cooked.
To use for an uncooked filling, simply bake for 20 minutes, remove the paper and beans and bake for a further 5 minutes until cooked and golden.