Peel, halve and quarter the quinces. If large, cut into thick segments. Gouge out the core and seeds with a teaspoon, rubbing with a cut lemon to stop discolouring.
Put water and golden caster sugar into a large saucepan and bring to the boil, add the quinces and let them cook for 15-25 minutes until easily pierced with a metal skewer.
Heat the oven to 180C/gas mark 4. Make the crumbs by creaming together butter and demerara sugar, then mix in plain flour, jumbo oats, salt flakes, and flaked almonds.
Scatter the crumb mixture over a baking tray and bake for 20-25 minutes until crisp.
Remove all but 200ml of the quince syrup from the saucepan, add butter, turn up the heat and let it reduce until the quinces are almost caramelised.
Spoon the quinces into a dish, smash the almond crumbs in a plastic bag, and scatter over the quinces.
