Preheat the oven to 190°C (375°F) (gas 5).
In a large bowl, cream together the butter and sugar using an electric hand whisk until the mixture is pale and fluffy.
Beat in the egg until combined.
Now, using a wooden spoon, stir in the oats, flour, baking powder, lemon zest, sultanas, and maple syrup.
Dust a work surface with a little flour and then make 12 even-sized balls from the mixture.
Place them on one large or two smaller baking sheets, spacing them well apart.
Bake for 12–15 minutes until golden in colour.
Leave to cool on the sheet for 5 minutes and then transfer to a wire rack to cool completely.