Used different ingredients.
I took a look at the recipe reviews for these muffins and incorporated suggestions. In summary, I made the recipe as printed, but I added 1/2 t. vanilla, replaced all of the milk with buttermilk and then added a few more tablespoons of buttermilk to the recipe, replaced the caster sugar in the streusel topping with demerara sugar, filled 12 regular-size muffin tins about 2/3 full and lightly placed the streusel on top. I then baked them in a 200c oven for 18 minutes. They were excellent! This recipe would be great with other types of fruit added (other berries, diced peaches, etc.) or with chocolate or white chocolate chips, etc.