Preheat oven to 180C and line 2 oven trays with baking paper. Combine flour, oats and sugar in a large bowl, stir in ½ tsp sea salt flakes and form a well in the centre.
Stir butter, golden syrup and 1 tbsp water in a saucepan over low heat until butter melts. Stir in bicarbonate of soda, then add to oat mixture and stir until mixture comes together. Set aside to cool, then roll between 2 sheets of baking paper until 4mm thick. Cut out 11cm-diameter rounds (re-roll scraps) and carefully transfer to prepared trays.
Bake in batches, swapping trays halfway through cooking, until biscuits are golden and crisp (12-14 minutes). Cool on trays for 5 minutes, then carefully transfer to a cooling rack.
Melt chocolate in a bowl over a saucepan of simmering water, stirring until smooth. Cool briefly, then dip one side of each biscuit in chocolate, shaking to remove excess. Place biscuits chocolate-side up on a wire rack until set. Hob-nobs will keep refrigerated in an airtight container or in a cool place for 5 days.