Preheat oven to 230 C/ 210 C fan and line a baking tray with a silicone baking mat.
Sift together the flour, salt, and caster sugar. Rub in the butter with your fingertips until it resembles coarse breadcrumbs.
Rub in the mashed potatoes until well combined.
Stir in the finely chopped herbs and make a well in the centre of the mixture.
Add the milk and stir well with a wooden spoon until just combined.
Turn out onto a floured board and knead for a moment until smooth.
Cut into 8 even sized pieces and form into small round bannock shapes.
Cut a cross into the top of each bannock and bake for 20-25 minutes until golden on top. Transfer to a wire rack to cool completely. Store in an airtight container until needed.