Preheat the oven to 220°C/200°Cfan/425°F/gas mark 7. Line a baking sheet with baking parchment. (You will need two baking sheets if you are making 12 smaller scones rather than 8 big ones).
Place the flour in a large bowl along with the baking powder, bicarbonate of soda, cinnamon, nutmeg, salt and sugar and mix to combine.
Add the cold, cubed butter/margarine and rub in using your fingertips until the mixture resembles fine breadcrumbs and no lumps of fat remain.
Stir through the chocolate chips.
Mash the bananas well with the vinegar and vanilla extract and add them to the bowl with about half of the milk.
Stir to moistent the flour and gradually add more milk as needed to form a soft, slightly sticky dough. Be careful not to add too much milk, the dough should be sticky but not wet.
Turn the dough out onto a well floured surface and bring it together into a rough ball. Don't knead it, just work it enough to bring it together. Dust with more flour and use your hands to pat it out into a round about 5cm/2in thick.
Use a well floured knife to slice the round into 8 wedges. Dip the knife in flour again between each cut to help prevent it from sticking.
If you want to make 12 smaller scones rather than 8 big ones then divide the dough in half, shape each half into a ball and pat out into a 5cm/2in round. Use a sharp, floured knife to cut each round into 6 wedges.
Gently place the wedges well spaced apart on the baking sheet(s). They will probably be a bit soft and delicate so it is a good idea to use a floured spatula or fish slice to help move them. If they get a bit squished out of shape you can gently pat them back into shape using floured hands once they are on the baking sheet.
Gently brush the tops of the scones with milk and sprinkle with demarera sugar. Bake for about 12-16 minutes until well risen, golden and firm to the touch.
Transfer to a wire rack and allow to cool a little before serving.