Combine the plain flour and caster sugar in a large bowl. Rub in the butter with your fingertips until the mixture resembles breadcrumbs (or you could use a pastry cutter if you have one).
Press about two-thirds of the shortbread mixture onto a lined baking tray (mine measured around 11 x 7 inches). Be sure to press it in firmly to help it hold together.
Scatter the blueberries across the shortbread, and sprinkle over the remaining breadcrumb-like shortbread mixture. Top with a couple of tablespoons of sugar, and bake for around 30 minutes, until the shortbread is light golden brown and some of the berries have released their juices.
Allow to cool completely, and then cut into squares with a pizza cutter.