Quince and Plum Crumble

Name: Quince and Plum Crumble
Yield: Serves 6
Ingredients: finely grated zest of 2 lemons and juice of ½; finely grated zest of 2 lemons and juice of ½; 100 g butter, cold and cut in small chunks; 100 g porridge oats; 50 g caster sugar; 50 g dark muscovado sugar; juice of ½ lemon; 175 g plain flour, sifted; butter, for greasing; 6 large ripe plums; 160 g caster sugar