Preheat the oven to 175℃ (350°F).
Grease a loaf tin (mine is 23cm x 12cm / 9" x 4¾") and line it with baking paper (I just use a strip as wide as the pan so that it is easy to lift the cake out).
In a medium bowl, combine the flour, baking powder and baking soda. Set aside.
In another bowl, whisk together the milk and lemon juice. Set aside.
Using an electric mixer or food processor, cream the butter and sugar until light and fluffy (around 3-4 minutes). Scrape down the sides of the bowl.
Beat the eggs in, one at a time. Scrape down the sides of the bowl again.
On a low speed, mix in around a quarter of the flour mixture. Follow this by adding one third of the milk mixture. Repeat these two steps, finishing with the flour. Finally, scrape down the sides of the bowl, and give it one last stir.
Pour half of the batter into the prepared pan and smooth out with a knife until fairly even.
Remove the strawberry swirl sauce from the fridge, take out 2 tablespoons and retain for later. Top the batter with around ¾ of the remaining sauce, and using a knife, swirl it into the batter.
Dollop the last of the batter on top, and again, smooth with a knife until even. Top with the last of the strawberry sauce and swirl that in, too.
Bake for around 45-50 mins. If you notice that the cake is browning too much on top, cover with some foil. The cake is ready when an inserted skewer comes out clean.
Allow the cake to cool in the pan for around 10 minutes, and then turn out on to a wire rack.