Start by mixing in the flour, bicarbonate, salt and oatmeal in a large mixing bowl. If using oats, make sure these are finely chopped or blended before adding to the mix.
Next, chop the butter into small pieces (easier to do if you've left the butter out of the fridge for an hour or so) then using your fingers gently rub the butter into the flour mixture.
Keep rubbing until the mixture resembles breadcrumbs, then add the sugar.
Slowly add the milk whilst stirring with a big spoon. Don't add it all at once - you're aiming for a reasonably solid dough which is workable and not too sticky.
Roll out the dough on a lightly floured surface until it is about an inch thick, then wrap it up in some cling film and pop it in the fridge. Feel free to split the dough into pieces if you don't have room in the fridge.
Leave the dough in the fridge until it is well chilled - at least half an hour, but an hour or longer is better.
Unwrap the dough and re-roll it until it is no thicker than 1cm. A thinner biscuit will have more of a crunch, whilst any thicker and it might be too soggy.
Use a cutter or knife to cut the biscuits into 8cm circles, then decorate with the knife however you like but make sure to add some holes with a fork.
Bake on a tray at 180°C or 350°F for 15 minutes.
Leave to cool on a rack then enjoy!