Heat the oven and line the tin. Preheat your oven to 170°C fan (190°C conventional, Gas Mark 5). Grease a 900 g loaf tin and line it with parchment paper, letting the paper overhang slightly at the long sides so you can lift the cake out later. The kitchen should already smell faintly warm as the oven comes up to temperature.

Cream the butter and sugar. In your mixer, beat the softened butter and caster sugar for 3–4 minutes until the mixture turns noticeably pale — almost the colour of clotted cream — and you can hear the gritty sugar dissolve into a soft, humming whir. Scrape down the bowl once with a spatula.

Add the eggs one at a time. Crack in the eggs one by one, beating for about 20 seconds after each. The batter may look slightly curdled after the third egg — that’s completely normal. Add the lemon zest and vanilla with the last egg and beat until the mixture looks glossy and smooth again.

Fold in the dry ingredients. Sift the flour, baking powder, and salt over the bowl. Pour in the milk and lemon juice. Using a large metal spoon, fold everything together with slow, sweeping strokes until you can no longer see streaks of flour. Stop the moment the batter looks uniform — overmixing is the enemy of a tender crumb.

Toss and fold in the blueberries. In a small bowl, gently toss the blueberries with the extra tablespoon of flour. This light dusting helps them stay suspended in the batter rather than sinking to the bottom. Fold them in with just two or three turns; the batter will streak with purple, which is exactly what you want.

Bake. Scrape the batter into the prepared tin and smooth the top with the back of your spoon. Bake on the middle shelf for 55–65 minutes. At the 45-minute mark, check the cake — if the top is browning too quickly, lay a piece of foil loosely over it. The cake is done when a skewer inserted into the centre comes out with just a few moist crumbs, and the top springs back when you press it gently with a fingertip.

Cool in the tin. Leave the cake to cool in the tin for 15 minutes. This rest is important — it lets the crumb set so the slices don’t crumble when you cut them. Then lift the cake out using the parchment overhang and transfer it to a wire rack to cool completely.

Make the glaze. Once the cake is cool, whisk the icing sugar and lemon juice together until you have a thick but pourable glaze — it should ribbon off the whisk and hold a faint trail for a second. Drizzle it over the top of the cake, letting it run down the sides in lazy drips.