Put the prunes, vanilla and water in a small saucepan. Simmer, uncovered, for 5 minutes until soft and pulpy and the water has been absorbed.
Heat the butter, sugar and syrup in a larger saucepan until melted. Stir in the flour, oats and seeds and mix until well combined.
Spoon three-quarters of the mixture into a 20 cm (8 inch) shallow square cake tin lined with nonstick baking paper (see
Leave to cool in the tin for 10 minutes then mark into 16 squares and leave to cool completely. Lift the flapjack out of the tin using the lining paper, peel off the paper and separate the squares. Store in an airtight tin for up to 3 days.