Mix the flours and the baking powder together, and the flaxseed if using ground.
Add the butter and cut in (or blitz in food processor) to breadcrumb texture.
Mix the seeds in.
Add the marmite and half the milk and stir in, and use hands to knead into a dough that holds together, a little crumbly is ok, or do this in the food processor until it starts to clump. Add more milk if required to make the dough clump.
Turn onto the work surface and pull the mixture into a dough.
Preheat the oven to 180C/160c Fan/350.
Line a couple of baking sheets with parchment paper.
Flatten the dough out and roll on a lightly floured surface to a rectangle 12x8 inches.
Cut one long side to make straight and use cut offs to complete the rectangle, patching gaps.
When you have created a rectangle divide the rectangle, long side, into 4 3inch strips.
Divide each 8x3 inch strip into 4 making 2x3 inch strips. That is the form of the biscuits.
Use the dull end of a wooden skewer, or other such implement, to poke a series of holes in the biscuits. (I made about 4 rows of 3 holes on each).
Transfer the biscuits to the baking sheets and bake for 20 to 25 minutes until baked through. They may feel soft when you take them out of the oven but should firm up and go crunchy as they cool. The longer you bake them the harder they will become.