In a large bowl, cream butter and sugar. Butter should be softened at room temperature to allow it to mix properly.
In a separate bowl, beat eggs until light. Add to sugar mixture.
Mash bananas and add lemon juice. Blend with creamed mixture.
Sift flour, baking powder and salt (omit if using salted butter) and mix with banana mixture.
Mix together until just combined.
Add walnuts.
Bake in greased loaf pan (or lined with parchment paper).
Bake at 375C for 1 1/4 hours.