Preheat the oven 175°C and grease and line a loaf tin with baking paper (about a 1 Litre capacity).
Place the orange and lemon zest with the caster sugar in the mixing bowl. Rub the zest into the sugar until most of the sugar has turned into a nice lemony colour (this infuses the sugar with the citrus oil).
Stir in the flour, cinnamon, salt and baking powder.
In a separate bowl, whisk together the oil, eggs and orange juice.
Beat the wet ingredients into the dry ingredients until you have a smooth batter.
Spoon a third of the batter into the loaf tin and scatter over about half the diced quince paste. Spoon another third of the batter, scatter some more quince paste over then pour the remaining mixture over the top.
Bake for 10 minutes then dot more quince paste over the top.
Bake for a further 30 minutes or until it's well risen and a skewer comes out clean when inserted into the cake.
Remove from oven and allow the cake to completely cool in the tin.
Dust the top with icing sugar and serve with a dollop of marscarpone and fresh orange slices. Store the cake in an airtight container (it'll keep for a week).