Grease and line the base of a 20cm square, shallow cake tin with butter and non-stick baking parchment. Heat the oven to 180C, fan oven 160C, gas mark 4.
Put the butter, sugar, syrup and cinnamon in a large saucepan and heat gently, stirring, until the butter has melted, and the sugar dissolved.
Take the pan off the heat and add the oats. Keep stirring until all the oats are thoroughly mixed in.
Tip about half the mixture into the lined tin and spread evenly to cover the base. Press down firmly and leave to cool for 2-3 minutes then dot the conserve over the top and spread evenly. Spoon on the remaining flapjack mixture and press down firmly over the conserve.
Bake for about 25-30 minutes or until the top is pale golden and slightly crispy. Leave to cool in the tin then cut into bite-sized pieces. Store in an airtight jar.
The flapjacks will keep in an airtight container for up to 1 week or in the freezer for 1 month.
Try Bonne Maman Apricot Conserve with a tablespoon of grated fresh ginger through the oats, or Black Cherry Conserve with 25g of dark chocolate grated on top.