Heat the butter, sugar and syrup gently until the butter has melted. Remove from the heat and mix in the oats, flour and bicarbonate of soda. Tip into a bowl and leave until cool enough to handle.
Shape the mixture into 12 balls. Space them well apart on a greased baking sheet and flatten each slightly. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 15 minutes.
Spread a teaspoonful of jam over the centre of each cookie. Thinly roll out the marzipan on a surface lightly dusted with icing sugar and cut out 12 rounds, using a 8 cm (3½ inch) round biscuit cutter. Use a 5 cm (2 inch) round cutter to cut out the centre of each round. Re-roll the trimmings and cut into 5 mm x 5 cm (¼ x 2 inch) strips. Arrange 4 strips over each cookie to make a diamond pattern and place a ring of marzipan over the top. Cook under a preheated grill for about 2 minutes until golden, watching closely as the marzipan will brown very quickly.
Transfer to a wire rack to cool and dust the edges of the cookies with icing sugar. They will keep, in an airtight container in a cool place, for a few days.