Unknown flower

Evening Primrose flowers are large, yellow and stunning. Having seen them around here growing wild in hedgerows I have wanted to get some growing in the garden. But it was not until last autumn that I was in a position to collect some seeds which I carefully put away in an envelope. Come spring I sowed them in a tray in my 'greenhouse' and surprisingly they all came up! I transplanted them into a couple of troughs and have been tending them on my terrace all summer. As there was no sign of flowers I looked them up - and of course they are bi-annual! So I just carried on watering them with the intention of planting them out in the autumn ready for next summer. However, suddenly, and from now where this flower appeared!
Isn't it stunning? Definitely not Evening Primrose and nothing like anything else we have in the garden so I have no idea where it has come from. However, I shall just enjoy, and collect the seeds for next year!