Thank you everyone, my mystery flower has been identified as Canterbury Bells.
In floriography, it represents gratitude, or faith and constancy.
The reason we didn’t see it last year from our box of mixed seed is because this biennial herbaceous plant forms rosettes of leaves in the first year, stems and flowers in the second one.
Apparently they can grow over 2 feet high and the corolla has five fused petals with lightly bent lobes (known as a coronate flower type).
The flowering period extends from May to July in the Northern Hemisphere. The flowers are either self-fertilized or pollinated by insects such as bees and butterflies. The seeds ripen from August to September and are dispersed by gravity alone.
The flowers have long-lasting blooms. These attractive bell-shaped flowers are short-stalked, large and bi-sexual with different shades of violet-blue or rarely white.
Ours is a white one! Photos below are mine.
Details extracted from Wiki (link)